diff --git a/Gemfile b/Gemfile
index c3efa379..231f2001 100644
--- a/Gemfile
+++ b/Gemfile
@@ -32,6 +32,13 @@ gem "sentry-rails"
 gem "turbolinks", "~> 5"
 gem "webrick"
 
+group :oidc do
+  # These are gems which are needed for OpenID connect. They are only required by the application
+  # when OIDC is enabled in the Postal configuration.
+  gem "omniauth_openid_connect"
+  gem "omniauth-rails_csrf_protection"
+end
+
 group :development, :assets do
   gem "coffee-rails", "~> 5.0"
   gem "jquery-rails"
diff --git a/Gemfile.lock b/Gemfile.lock
index c6b997b3..0f4fe96b 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -68,16 +68,20 @@ GEM
       tzinfo (~> 2.0)
     addressable (2.8.6)
       public_suffix (>= 2.0.2, < 6.0)
+    aes_key_wrap (1.1.0)
     annotate (3.2.0)
       activerecord (>= 3.2, < 8.0)
       rake (>= 10.4, < 14.0)
     ast (2.4.2)
+    attr_required (1.0.2)
     authie (4.1.3)
       activerecord (>= 6.1, < 8.0)
     autoprefixer-rails (10.4.13.0)
       execjs (~> 2)
+    base64 (0.2.0)
     bcrypt (3.1.20)
     bigdecimal (3.1.6)
+    bindata (2.5.0)
     builder (3.2.4)
     chronic (0.10.2)
     coffee-rails (5.0.0)
@@ -106,6 +110,8 @@ GEM
     dynamic_form (1.3.1)
       actionview (> 5.2.0)
       activemodel (> 5.2.0)
+    email_validator (2.2.4)
+      activemodel
     encrypto_signo (1.0.0)
     erubi (1.12.0)
     execjs (2.7.0)
@@ -114,6 +120,12 @@ GEM
     factory_bot_rails (6.4.3)
       factory_bot (~> 6.4)
       railties (>= 5.0.0)
+    faraday (2.9.0)
+      faraday-net_http (>= 2.0, < 3.2)
+    faraday-follow_redirects (0.3.0)
+      faraday (>= 1, < 3)
+    faraday-net_http (3.1.0)
+      net-http
     ffi (1.15.5)
     gelf (3.1.0)
       json
@@ -133,6 +145,13 @@ GEM
       railties (>= 4.2.0)
       thor (>= 0.14, < 2.0)
     json (2.7.1)
+    json-jwt (1.16.6)
+      activesupport (>= 4.2)
+      aes_key_wrap
+      base64
+      bindata
+      faraday (~> 2.0)
+      faraday-follow_redirects
     kaminari (1.2.2)
       activesupport (>= 4.1.0)
       kaminari-actionview (= 1.2.2)
@@ -169,6 +188,8 @@ GEM
       json
       rack (>= 1.4)
     mysql2 (0.5.6)
+    net-http (0.4.1)
+      uri
     net-imap (0.4.10)
       date
       net-protocol
@@ -194,6 +215,29 @@ GEM
       racc (~> 1.4)
     nokogiri (1.16.2-x86_64-linux)
       racc (~> 1.4)
+    omniauth (2.1.2)
+      hashie (>= 3.4.6)
+      rack (>= 2.2.3)
+      rack-protection
+    omniauth-rails_csrf_protection (1.0.1)
+      actionpack (>= 4.2)
+      omniauth (~> 2.0)
+    omniauth_openid_connect (0.7.1)
+      omniauth (>= 1.9, < 3)
+      openid_connect (~> 2.2)
+    openid_connect (2.3.0)
+      activemodel
+      attr_required (>= 1.0.0)
+      email_validator
+      faraday (~> 2.0)
+      faraday-follow_redirects
+      json-jwt (>= 1.16)
+      mail
+      rack-oauth2 (~> 2.2)
+      swd (~> 2.0)
+      tzinfo
+      validate_url
+      webfinger (~> 2.0)
     parallel (1.22.1)
     parser (3.2.1.1)
       ast (~> 2.4.1)
@@ -203,6 +247,16 @@ GEM
       nio4r (~> 2.0)
     racc (1.7.3)
     rack (2.2.8.1)
+    rack-oauth2 (2.2.1)
+      activesupport
+      attr_required
+      faraday (~> 2.0)
+      faraday-follow_redirects
+      json-jwt (>= 1.11.0)
+      rack (>= 2.1.0)
+    rack-protection (3.2.0)
+      base64 (>= 0.1.0)
+      rack (~> 2.2, >= 2.2.4)
     rack-test (2.1.0)
       rack (>= 1.3)
     rails (7.0.8.1)
@@ -302,6 +356,11 @@ GEM
       actionpack (>= 5.2)
       activesupport (>= 5.2)
       sprockets (>= 3.0.0)
+    swd (2.0.3)
+      activesupport (>= 3)
+      attr_required (>= 0.0.5)
+      faraday (~> 2.0)
+      faraday-follow_redirects
     temple (0.10.3)
     thor (1.3.0)
     tilt (2.3.0)
@@ -315,6 +374,14 @@ GEM
     uglifier (4.2.0)
       execjs (>= 0.3.0, < 3)
     unicode-display_width (2.4.2)
+    uri (0.13.0)
+    validate_url (1.0.15)
+      activemodel (>= 3.0.0)
+      public_suffix
+    webfinger (2.1.3)
+      activesupport
+      faraday (~> 2.0)
+      faraday-follow_redirects
     webmock (3.20.0)
       addressable (>= 2.8.0)
       crack (>= 0.3.2)
@@ -360,6 +427,8 @@ DEPENDENCIES
   nifty-utils
   nilify_blanks
   nio4r
+  omniauth-rails_csrf_protection
+  omniauth_openid_connect
   prometheus-client
   puma
   rails (= 7.0.8.1)
diff --git a/app/assets/stylesheets/application/components/_login_form.scss b/app/assets/stylesheets/application/components/_login_form.scss
index 030b0567..16de08a6 100644
--- a/app/assets/stylesheets/application/components/_login_form.scss
+++ b/app/assets/stylesheets/application/components/_login_form.scss
@@ -1,22 +1,32 @@
-.loginForm {
-
-}
+.loginForm {}
 
 
 .loginForm__input {
-  margin-bottom:15px;
+  margin-bottom: 15px;
 }
 
 .loginForm__submit {
-  display:flex;
-  justify-content:space-between;
-  align-items:center;
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
 }
 
 
 .loginForm__links {
-  font-size:12px;
-  color:#999;
+  font-size: 12px;
+  color: #999;
   text-decoration: underline;
-  line-height:1.7;
+  line-height: 1.7;
+}
+
+.loginForm__oidcButton {
+  margin-bottom: 15px;
+  padding-bottom: 25px;
+  border-bottom: 1px solid #e4e8ef;
+}
+
+.loginForm__localTitle {
+  text-align: center;
+  margin-bottom: 15px;
+  color: #999;
 }
diff --git a/app/assets/stylesheets/application/components/_user_list.scss b/app/assets/stylesheets/application/components/_user_list.scss
index f6e315ba..c3eeabf6 100644
--- a/app/assets/stylesheets/application/components/_user_list.scss
+++ b/app/assets/stylesheets/application/components/_user_list.scss
@@ -1,74 +1,67 @@
 .userList {
-  border-radius:4px;
-  color:$darkBlue;
-  overflow:hidden;
-  box-shadow:0 0 10px rgba(0,0,0,0.2);
+  border-radius: 4px;
+  color: $darkBlue;
+  overflow: hidden;
+  box-shadow: 0 0 10px rgba(0, 0, 0, 0.2);
 }
 
 .userList__item {
-  display:block;
-  background:#fff;
-  padding:15px;
-  display:flex;
+  display: block;
+  background: #fff;
+  padding: 15px;
+  display: flex;
   align-items: center;
 }
+
 .userList__item:nth-child(even) {
-  background:none;
+  background: none;
 }
 
-.userList__item + .userList__item {
-  border-top:1px solid lighten(#ccd4e0, 10%);
-}
-
-.userList__avatar {
-  width:50px;
-  height:50px;
-  border-radius:50%;
-  background:#fff;
-  border:2px solid #efefef;
-  padding:3px;
-  flex: 0 0 auto;
+.userList__item+.userList__item {
+  border-top: 1px solid lighten(#ccd4e0, 10%);
 }
 
 .userList__details {
   flex: 1 1 auto;
-  margin:0 25px;
+  margin: 0 0;
 }
 
 
 .userList__actions {
   flex: 0 0 auto;
-  width:180px;
-  font-size:12px;
-  line-height:1.5;
-  color:#999;
-  text-decoration: underline;
+  width: 120px;
+  font-size: 12px;
+  line-height: 1.5;
+  color: #999;
+
+  a {
+    text-decoration: underline;
+  }
 }
 
 .userList__name {
-  font-weight:600;
-  font-size:16px;
-  margin-bottom:3px;
+  font-weight: 600;
+  font-size: 16px;
+  margin-bottom: 3px;
 }
 
 .userList__owner {
-  vertical-align:2px;
-  margin-left:5px;
-  background-color:$orange;
+  vertical-align: 2px;
+  margin-left: 5px;
+  background-color: $orange;
 }
 
 .userList__pending {
-  vertical-align:2px;
-  margin-left:5px;
-  background-color:#ccc;
+  vertical-align: 2px;
+  margin-left: 5px;
+  background-color: #ccc;
 }
 
-.userList__admin {
-  vertical-align:2px;
-  margin-left:5px;
-  background-color:$blue;
+.userList__tag {
+  vertical-align: 2px;
+  margin-left: 3px;
 }
 
 .userList__revoke {
-  color:$red;
+  color: $red;
 }
diff --git a/app/assets/stylesheets/application/elements/_button.scss b/app/assets/stylesheets/application/elements/_button.scss
index b66116a1..0ea514d6 100644
--- a/app/assets/stylesheets/application/elements/_button.scss
+++ b/app/assets/stylesheets/application/elements/_button.scss
@@ -1,98 +1,117 @@
 .button {
-  display:inline-block;
-  font:inherit;
-  border-radius:4px;
-  appearance:none;
-  background:$blue;
-  color:#fff;
-  font-size:14px !important;
-  margin:0;
-  vertical-align:top;
-  padding:6px 15px;
-  border:2px solid transparent;
-  border-bottom:2px solid darken($blue, 20%);
+  display: inline-block;
+  font: inherit;
+  border-radius: 4px;
+  appearance: none;
+  background: $blue;
+  color: #fff;
+  font-size: 14px !important;
+  margin: 0;
+  vertical-align: top;
+  padding: 6px 15px;
+  border: 2px solid transparent;
+  border-bottom: 2px solid darken($blue, 20%);
+
   &:active {
-    background-color:darken($blue, 15%);
+    background-color: darken($blue, 15%);
   }
+
   &:focus {
-    border-color:darken($blue, 15%);
-    background-color:lighten($blue, 5%);
+    border-color: darken($blue, 15%);
+    background-color: lighten($blue, 5%);
   }
+
   &.is-spinning {
-    color:transparent;
-    background-repeat:no-repeat;
+    color: transparent;
+    background-repeat: no-repeat;
     background-position: center center;
-    background-size:25px;
-    background-image:image-url('button-spinner.gif');
+    background-size: 25px;
+    background-image: image-url('button-spinner.gif');
   }
 }
 
 .button--small {
-  font-size:12px !important;
-  padding:3px 10px;
-  border-width:1px;
+  font-size: 12px !important;
+  padding: 3px 10px;
+  border-width: 1px;
 }
 
 .button--positive {
-  background-color:$green;
-  border-bottom-color:darken($green, 15%);
+  background-color: $green;
+  border-bottom-color: darken($green, 15%);
+
   &:active {
-    background-color:darken($green, 15%);
+    background-color: darken($green, 15%);
   }
+
   &:focus {
-    border-color:darken($green, 15%);
-    background-color:lighten($green, 5%);
+    border-color: darken($green, 15%);
+    background-color: lighten($green, 5%);
   }
+
   &.is-spinning {
-    background-image:image-url('button-spinner-positive.gif');
+    background-image: image-url('button-spinner-positive.gif');
   }
 
 }
 
 
 .button--neutral {
-  background-color:#ccc;
-  border-bottom-color:darken(#ccc, 15%);
+  background-color: #ccc;
+  border-bottom-color: darken(#ccc, 15%);
+
   &:active {
-    background-color:darken(#ccc, 15%);
+    background-color: darken(#ccc, 15%);
   }
+
   &:focus {
-    border-color:darken(#ccc, 15%);
-    background-color:lighten(#ccc, 5%);
+    border-color: darken(#ccc, 15%);
+    background-color: lighten(#ccc, 5%);
   }
+
   &.is-spinning {
-    background-image:image-url('button-spinner-neutral.gif');
+    background-image: image-url('button-spinner-neutral.gif');
   }
 }
 
 .button--danger {
-  background-color:$red;
-  border-bottom-color:darken($red, 15%);
+  background-color: $red;
+  border-bottom-color: darken($red, 15%);
+
   &:active {
-    background-color:darken($red, 15%);
+    background-color: darken($red, 15%);
   }
+
   &:focus {
-    border-color:darken($red, 15%);
-    background-color:lighten($red, 5%);
+    border-color: darken($red, 15%);
+    background-color: lighten($red, 5%);
   }
+
   &.is-spinning {
-    background-image:image-url('button-spinner-danger.gif');
+    background-image: image-url('button-spinner-danger.gif');
   }
 }
 
 .button--dark {
-  background-color:$darkBlue;
-  border-bottom-color:darken($darkBlue, 15%);
+  background-color: $darkBlue;
+  border-bottom-color: darken($darkBlue, 15%);
+
   &:active {
-    background-color:darken($darkBlue, 15%);
+    background-color: darken($darkBlue, 15%);
   }
+
   &:focus {
-    border-color:darken($darkBlue, 15%);
-    background-color:lighten($darkBlue, 5%);
+    border-color: darken($darkBlue, 15%);
+    background-color: lighten($darkBlue, 5%);
   }
+
   &.is-spinning {
-    background-image:image-url('button-spinner-dark.gif');
+    background-image: image-url('button-spinner-dark.gif');
   }
 
 }
 
+.button--full {
+  width: 100%;
+  text-align: center;
+}
diff --git a/app/assets/stylesheets/application/elements/_label.scss b/app/assets/stylesheets/application/elements/_label.scss
index e8752633..b769c746 100644
--- a/app/assets/stylesheets/application/elements/_label.scss
+++ b/app/assets/stylesheets/application/elements/_label.scss
@@ -1,136 +1,137 @@
 .label {
-  display:inline-block;
-  background:#000;
-  color:#fff;
-  font-size:9px;
+  display: inline-block;
+  background: #000;
+  color: #fff;
+  font-size: 9px;
   text-transform: uppercase;
-  border-radius:40px;
-  padding:2px 6px;
-  line-height:0.9;
+  border-radius: 40px;
+  padding: 2px 6px;
+  line-height: 0.9;
 }
 
 .label--green {
-  background-color:$green;
+  background-color: $green;
 }
 
 .label--red {
-  background-color:$red;
+  background-color: $red;
 }
 
 .label--orange {
-  background-color:$orange;
+  background-color: $orange;
 }
 
 .label--blue {
-  background-color:$blue;
+  background-color: $blue;
 }
 
 .label--grey {
-  background-color:#999;
+  background-color: #999;
 }
 
 .label--turquoise {
-  background-color:$blue;
+  background-color: $blue;
 }
 
 .label--purple {
-  background-color:$purple;
+  background-color: $purple;
 }
 
 .label--large {
-  font-size:11px;
-  padding:4px 10px;
+  font-size: 11px;
+  padding: 4px 10px;
 }
 
 .label--serverStatus-live {
-  background-color:$green;
+  background-color: $green;
 }
 
 .label--serverStatus-development {
-  background-color:#636363;
+  background-color: #636363;
 }
 
 .label--serverStatus-suspended {
-  background-color:$red;
+  background-color: $red;
 }
 
 .label--messageStatus-pending {
-  background-color:$subBlue;
+  background-color: $subBlue;
 }
 
 .label--messageStatus-held {
-  background-color:#aaa;
+  background-color: #aaa;
 }
 
 .label--messageStatus-processed {
-  background-color:$green;
+  background-color: $green;
 }
 
 .label--messageStatus-sent {
-  background-color:$green;
+  background-color: $green;
 }
 
 .label--messageStatus-hard_fail {
-  background-color:$red;
+  background-color: $red;
 }
 
 .label--messageStatus-soft_fail {
-  background-color:$orange;
+  background-color: $orange;
 }
 
 .label--messageStatus-bounced {
-  background-color:$red;
+  background-color: $red;
 }
 
 .label--messageStatus-hold_cancelled {
-  background-color:#ccc;
+  background-color: #ccc;
 }
 
 
 .label--credentialType-api {
-  background-color:$blue;
+  background-color: $blue;
 }
 
 .label--credentialType-smtp {
-  background-color:$turquoise;
+  background-color: $turquoise;
 }
 
 .label--credentialType-smtp_ip {
-  background-color:$orange;
+  background-color: $orange;
 }
 
 .label--spamStatus-not_checked {
-  background:#aaa;
+  background: #aaa;
 }
 
 .label--spamStatus-spam {
-  background:$orange;
+  background: $orange;
 }
 
 .label--spamStatus-not_spam {
-  background:$turquoise;
+  background: $turquoise;
 }
 
 .label--http-status-2 {
-  background-color:$green;
+  background-color: $green;
 }
 
 .label--http-status-3 {
-  background-color:$orange;
+  background-color: $orange;
 }
 
 .label--http-status-4,
 .label--http-status-5 {
-  background-color:$red;
+  background-color: $red;
 }
 
 .domainList__ssl {
-  color:$green;
+  color: $green;
+
   &:hover {
-    text-decoration:underline;
+    text-decoration: underline;
   }
 }
 
 .domainList__ssl--disabled {
-  color:#999;
+  color: #999;
 }
diff --git a/app/controllers/sessions_controller.rb b/app/controllers/sessions_controller.rb
index 99a80f21..19026a05 100644
--- a/app/controllers/sessions_controller.rb
+++ b/app/controllers/sessions_controller.rb
@@ -4,7 +4,7 @@ class SessionsController < ApplicationController
 
   layout "sub"
 
-  skip_before_action :login_required, only: [:new, :create, :begin_password_reset, :finish_password_reset, :ip, :raise_error]
+  skip_before_action :login_required, only: [:new, :create, :begin_password_reset, :finish_password_reset, :ip, :raise_error, :create_from_oidc, :oauth_failure]
 
   def create
     login(User.authenticate(params[:email_address], params[:password]))
@@ -29,12 +29,16 @@ def persist
   def begin_password_reset
     return unless request.post?
 
-    if user = User.where(email_address: params[:email_address]).first
-      user.begin_password_reset(params[:return_to])
-      redirect_to login_path(return_to: params[:return_to]), notice: "Please check your e-mail and click the link in the e-mail we've sent you."
-    else
-      redirect_to login_reset_path(return_to: params[:return_to]), alert: "No user exists with that e-mail address. Please check and try again."
+    user_scope = Postal::Config.oidc.enabled? ? User.with_password : User
+    user = user_scope.find_by(email_address: params[:email_address])
+
+    if user.nil?
+      redirect_to login_reset_path(return_to: params[:return_to]), alert: "No local user exists with that e-mail address. Please check and try again."
+      return
     end
+
+    user.begin_password_reset(params[:return_to])
+    redirect_to login_path(return_to: params[:return_to]), notice: "Please check your e-mail and click the link in the e-mail we've sent you."
   end
 
   def finish_password_reset
@@ -49,6 +53,7 @@ def finish_password_reset
       flash.now[:alert] = "You must enter a new password"
       return
     end
+
     @user.password = params[:password]
     @user.password_confirmation = params[:password_confirmation]
     return unless @user.save
@@ -61,4 +66,25 @@ def ip
     render plain: "ip: #{request.ip} remote ip: #{request.remote_ip}"
   end
 
+  def create_from_oidc
+    unless Postal::Config.oidc.enabled?
+      raise Postal::Error, "OIDC cannot be used unless enabled in the configuration"
+    end
+
+    auth = request.env["omniauth.auth"]
+    user = User.find_from_oidc(auth.extra.raw_info, logger: Postal.logger)
+    if user.nil?
+      redirect_to login_path, alert: "No user was found matching your identity. Please contact your administrator."
+      return
+    end
+
+    login(user)
+    flash[:remember_login] = true
+    redirect_to_with_return_to root_path
+  end
+
+  def oauth_failure
+    redirect_to login_path, alert: "An issue occurred while logging you in with OpenID. Please try again later or contact your administrator."
+  end
+
 end
diff --git a/app/controllers/user_controller.rb b/app/controllers/user_controller.rb
index a9af998a..ff6cd5ae 100644
--- a/app/controllers/user_controller.rb
+++ b/app/controllers/user_controller.rb
@@ -30,23 +30,28 @@ def create
 
   def update
     @user = User.find(current_user.id)
-    @user.attributes = params.require(:user).permit(:first_name, :last_name, :time_zone, :email_address, :password, :password_confirmation)
-
-    if @user.authenticate_with_previous_password_first(params[:password])
-      @password_correct = true
-    else
-      respond_to do |wants|
-        wants.html do
-          flash.now[:alert] = "The current password you have entered is incorrect. Please check and try again."
-          render "edit"
-        end
-        wants.json do
-          render json: { alert: "The current password you've entered is incorrect. Please check and try again" }
+    safe_params = [:first_name, :last_name, :time_zone, :email_address]
+
+    if @user.password?
+      safe_params += [:password, :password_confirmation]
+      if @user.authenticate_with_previous_password_first(params[:password])
+        @password_correct = true
+      else
+        respond_to do |wants|
+          wants.html do
+            flash.now[:alert] = "The current password you have entered is incorrect. Please check and try again."
+            render "edit"
+          end
+          wants.json do
+            render json: { alert: "The current password you've entered is incorrect. Please check and try again" }
+          end
         end
+        return
       end
-      return
     end
 
+    @user.attributes = params.require(:user).permit(safe_params)
+
     if @user.save
       redirect_to_with_json settings_path, notice: "Your settings have been updated successfully."
     else
diff --git a/app/models/concerns/has_authentication.rb b/app/models/concerns/has_authentication.rb
index 17c06d2c..c19bc7b1 100644
--- a/app/models/concerns/has_authentication.rb
+++ b/app/models/concerns/has_authentication.rb
@@ -5,15 +5,20 @@ module HasAuthentication
   extend ActiveSupport::Concern
 
   included do
-    has_secure_password
+    has_secure_password validations: false
 
     validates :password, length: { minimum: 8, allow_blank: true }
+    validates :password, confirmation: { allow_blank: true }
+    validate :validate_password_presence
+
     before_save :clear_password_reset_token_on_password_change
+
+    scope :with_password, -> { where.not(password_digest: nil) }
   end
 
   class_methods do
     def authenticate(email_address, password)
-      user = where(email_address: email_address).first
+      user = find_by(email_address: email_address)
       raise Postal::Errors::AuthenticationError, "InvalidEmailAddress" if user.nil?
       raise Postal::Errors::AuthenticationError, "InvalidPassword" unless user.authenticate(password)
 
@@ -30,6 +35,10 @@ def authenticate_with_previous_password_first(unencrypted_password)
   end
 
   def begin_password_reset(return_to = nil)
+    if Postal::Config.oidc.enabled? && (oidc_uid.present? || password_digest.blank?)
+      raise Postal::Error, "User has OIDC enabled, password resets are not supported"
+    end
+
     self.password_reset_token = SecureRandom.alphanumeric(24)
     self.password_reset_token_valid_until = 1.day.from_now
     save!
@@ -45,6 +54,12 @@ def clear_password_reset_token_on_password_change
     self.password_reset_token_valid_until = nil
   end
 
+  def validate_password_presence
+    return if password_digest.present? || Postal::Config.oidc.enabled?
+
+    errors.add :password, :blank
+  end
+
 end
 
 # -*- SkipSchemaAnnotations
diff --git a/app/models/user.rb b/app/models/user.rb
index b2831f3c..b22926ea 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -5,19 +5,21 @@
 # Table name: users
 #
 #  id                               :integer          not null, primary key
-#  uuid                             :string(255)
+#  admin                            :boolean          default(FALSE)
+#  email_address                    :string(255)
+#  email_verification_token         :string(255)
+#  email_verified_at                :datetime
 #  first_name                       :string(255)
 #  last_name                        :string(255)
-#  email_address                    :string(255)
+#  oidc_issuer                      :string(255)
+#  oidc_uid                         :string(255)
 #  password_digest                  :string(255)
+#  password_reset_token             :string(255)
+#  password_reset_token_valid_until :datetime
 #  time_zone                        :string(255)
-#  email_verification_token         :string(255)
-#  email_verified_at                :datetime
+#  uuid                             :string(255)
 #  created_at                       :datetime
 #  updated_at                       :datetime
-#  password_reset_token             :string(255)
-#  password_reset_token_valid_until :datetime
-#  admin                            :boolean          default(FALSE)
 #
 # Indexes
 #
@@ -28,13 +30,11 @@
 class User < ApplicationRecord
 
   include HasUUID
-
   include HasAuthentication
 
   validates :first_name, presence: true
   validates :last_name, presence: true
   validates :email_address, presence: true, uniqueness: { case_sensitive: false }, format: { with: /@/, allow_blank: true }
-  validates :time_zone, presence: true
 
   default_value :time_zone, -> { "UTC" }
 
@@ -53,24 +53,83 @@ def name
     "#{first_name} #{last_name}"
   end
 
-  def to_param
-    uuid
+  def password?
+    password_digest.present?
   end
 
-  def md5_for_gravatar
-    @md5_for_gravatar ||= Digest::MD5.hexdigest(email_address.to_s.downcase)
+  def oidc?
+    oidc_uid.present?
   end
 
-  def avatar_url
-    @avatar_url ||= email_address ? "https://secure.gravatar.com/avatar/#{md5_for_gravatar}?rating=PG&size=120&d=mm" : nil
+  def to_param
+    uuid
   end
 
   def email_tag
     "#{name} <#{email_address}>"
   end
 
-  def self.[](email)
-    where(email_address: email).first
+  class << self
+
+    # Lookup a user by email address
+    #
+    # @param email [String] the email address
+    #
+    # @return [User, nil] the user
+    def [](email)
+      find_by(email_address: email)
+    end
+
+    # Find a user based on an OIDC authentication hash
+    #
+    # @param auth [Hash] the authentication hash
+    # @param logger [Logger] a logger to log debug information to
+    #
+    # @return [User, nil] the user
+    def find_from_oidc(auth, logger: nil)
+      config = Postal::Config.oidc
+
+      uid = auth[config.uid_field]
+      oidc_name = auth[config.name_field]
+      oidc_email_address = auth[config.email_address_field]
+
+      # look for an existing user with the same UID and OIDC issuer. If we find one,
+      # this is the user we'll want to use.
+      user = where(oidc_uid: uid, oidc_issuer: config.issuer).first
+
+      if user
+        logger&.debug "found user with UID #{uid} for issuer #{config.issuer} (user ID: #{user.id})"
+      else
+        logger&.debug "no user with UID #{uid} for issuer #{config.issuer}"
+      end
+
+      # if we don't have an existing user, we will look for users which have no OIDC
+      # credentials but with a matching e-mail address.
+      if user.nil? && oidc_email_address.present?
+        user = where(oidc_uid: nil, email_address: oidc_email_address).first
+        if user
+          logger&.debug "found user with e-mail address #{oidc_email_address} (user ID: #{user.id})"
+        else
+          logger&.debug "no user with e-mail address #{oidc_email_address}"
+        end
+      end
+
+      # now, if we still don't have a user, we're not going to create one so we'll just
+      # return nil (we might auto create users in the future but not right now)
+      return if user.nil?
+
+      # otherwise, let's update our user as appropriate
+      user.oidc_uid = uid
+      user.oidc_issuer = config.issuer
+      user.email_address = oidc_email_address if oidc_email_address.present?
+      user.first_name, user.last_name = oidc_name.split(/\s+/, 2) if oidc_name.present?
+      user.password = nil
+      user.save!
+
+      # return the user
+      user
+    end
+
   end
 
 end
diff --git a/app/views/sessions/new.html.haml b/app/views/sessions/new.html.haml
index bab54527..f4410745 100644
--- a/app/views/sessions/new.html.haml
+++ b/app/views/sessions/new.html.haml
@@ -6,13 +6,15 @@
 .subPageBox__content
   = form_tag login_path, :class => 'loginForm' do
     = hidden_field_tag 'return_to', params[:return_to]
-    - if params[:return_to] && params[:return_to] =~ /\/join\//
-      %p.loginForm__invite.warningBox.u-margin Login to your existing account to accept your invitation.
 
-    %p.loginForm__input= text_field_tag 'email_address', '', :type => 'email', :autocomplete => 'off', :spellcheck => 'false', :class => 'input input--text input--onWhite', :placeholder => "Your e-mail address", :autofocus => true, :tabindex => 1
+    - if Postal::Config.oidc.enabled?
+      .loginForm__oidcButton
+        = link_to "Login with #{Postal::Config.oidc.name}", "/auth/oidc", method: :post, class: 'button button--full'
+
+    %p.loginForm__localTitle or login with a local user
+    %p.loginForm__input= text_field_tag 'email_address', '', :type => 'email', :spellcheck => 'false', :class => 'input input--text input--onWhite', :placeholder => "Your e-mail address", :autofocus => !Postal::Config.oidc.enabled?, :tabindex => 1
     %p.loginForm__input= password_field_tag 'password', '', :class => 'input input--text input--onWhite', :placeholder => "Your password", :tabindex => 2
     .loginForm__submit
       %ul.loginForm__links
         %li= link_to "Forgotten your password?", login_reset_path(:return_to => params[:return_to])
       %p= submit_tag "Login", :class => 'button button--positive', :tabindex => 3
-
diff --git a/app/views/user/edit.html.haml b/app/views/user/edit.html.haml
index 53d2fa24..48e136be 100644
--- a/app/views/user/edit.html.haml
+++ b/app/views/user/edit.html.haml
@@ -6,15 +6,16 @@
   = form_for @user, :url => settings_path, :remote => true do |f|
     = f.error_messages
     %fieldset.fieldSet
-      .fieldSet__field
-        = label_tag :password, 'Your Password', :class => 'fieldSet__label'
-        .fieldSet__input
-          = password_field_tag :password, params[:password], :autofocus => @password_correct.nil?, :disabled => @password_correct, :class => 'input input--text', :placeholder => "Enter your current password to change your details"
-          - if @password_correct
-            = hidden_field_tag :password, params[:password]
-          %p.fieldSet__text
-            In order to protect your account, you need to enter your current password in the field above
-            to authenticate the change of your details.
+      - if @user.password?
+        .fieldSet__field
+          = label_tag :password, 'Your Password', :class => 'fieldSet__label'
+          .fieldSet__input
+            = password_field_tag :password, params[:password], :autofocus => @password_correct.nil?, :disabled => @password_correct, :class => 'input input--text', :placeholder => "Enter your current password to change your details"
+            - if @password_correct
+              = hidden_field_tag :password, params[:password]
+            %p.fieldSet__text
+              In order to protect your account, you need to enter your current password in the field above
+              to authenticate the change of your details.
 
       .fieldSet__title
         Your details
@@ -41,14 +42,15 @@
             Choose the time zone that you'd like times to be displayed to you when you use our
             web interface. By default, times are displayed in UTC.
 
-      .fieldSet__title
-        Change your password?
-      .fieldSet__field
-        = f.label :password, "New Password", :class => 'fieldSet__label'
-        .fieldSet__input
-          .inputPair
-            = f.password_field :password, :class => 'input input--text', :placeholder => "•••••••••••", :value => @user.password
-            = f.password_field :password_confirmation, :class => 'input input--text', :placeholder => "and confirm it", :value => @user.password_confirmation
+      - if @user.password?
+        .fieldSet__title
+          Change your password?
+        .fieldSet__field
+          = f.label :password, "New Password", :class => 'fieldSet__label'
+          .fieldSet__input
+            .inputPair
+              = f.password_field :password, :class => 'input input--text', :placeholder => "•••••••••••", :value => @user.password
+              = f.password_field :password_confirmation, :class => 'input input--text', :placeholder => "and confirm it", :value => @user.password_confirmation
 
 
     %p.fieldSetSubmit.buttonSet
diff --git a/app/views/users/_form.html.haml b/app/views/users/_form.html.haml
index 0011fb14..f7e89ef1 100644
--- a/app/views/users/_form.html.haml
+++ b/app/views/users/_form.html.haml
@@ -15,7 +15,15 @@
     - unless @user.persisted?
       .fieldSet__field
         = f.label :password, :class => 'fieldSet__label'
-        .fieldSet__input= f.password_field :password, :class => 'input input--text', :placeholder => '•••••••••••', autocomplete: 'one-time-code'
+        .fieldSet__input
+          = f.password_field :password, :class => 'input input--text', :placeholder => '•••••••••••', autocomplete: 'one-time-code'
+          - if Postal::Config.oidc.enabled?
+            %p.fieldSet__text
+              You have enabled OIDC which means a password is not required. If you do not provide
+              a password this user will be matched to an OIDC identity based on the e-mail address
+              provided above. You may, however, enter a password and this user will be permitted to
+              use that password until they have successfully logged in with OIDC.
+        
       .fieldSet__field
         = f.label :password_confirmation, "Confirm".html_safe, :class => 'fieldSet__label'
         .fieldSet__input= f.password_field :password_confirmation, :class => 'input input--text', :placeholder => '•••••••••••', autocomplete: 'one-time-code'
diff --git a/app/views/users/index.html.haml b/app/views/users/index.html.haml
index 73c28f32..6d31da9c 100644
--- a/app/views/users/index.html.haml
+++ b/app/views/users/index.html.haml
@@ -7,15 +7,16 @@
   %ul.userList.u-margin
     - for user in @users
       %li.userList__item
-        = image_tag user.avatar_url, :class => 'userList__avatar'
         .userList__details
           %p.userList__name
             = user.name
             - if user.admin?
-              %span.userList__admin.label Admin
+              %span.userList__tag.label.label--blue Admin
+            - if Postal::Config.oidc.enabled? && user.oidc?
+              %span.userList__tag.label.label--green OIDC
           %p.userList__email= user.email_address
         %ul.userList__actions
-          %li= link_to "Edit permissions", [:edit, user]
+          %li= link_to "Edit user", [:edit, user]
           %li= link_to "Delete user", user, :method => :delete, :data => {:confirm => "Are you sure you wish to revoke #{user.name}'s access?", :disable_with => "Deleting..."}, :remote => true, :class => 'userList__revoke'
 
   %p.u-center= link_to "Add a new user", :new_user, :class => 'button button--positive'
diff --git a/config/application.rb b/config/application.rb
index a52bee33..ad17d2f8 100644
--- a/config/application.rb
+++ b/config/application.rb
@@ -12,7 +12,9 @@
 
 # Require the gems listed in Gemfile, including any gems
 # you've limited to :test, :development, or :production.
-Bundler.require(*Rails.groups)
+gem_groups = Rails.groups
+gem_groups << :oidc if Postal::Config.oidc.enabled?
+Bundler.require(*gem_groups)
 
 module Postal
   class Application < Rails::Application
diff --git a/config/initializers/inflections.rb b/config/initializers/inflections.rb
index 3a68b273..c4efa423 100644
--- a/config/initializers/inflections.rb
+++ b/config/initializers/inflections.rb
@@ -16,6 +16,7 @@
 ActiveSupport::Inflector.inflections(:en) do |inflect|
   inflect.acronym "DKIM"
   inflect.acronym "HTTP"
+  inflect.acronym "OIDC"
   inflect.acronym "SMTP"
   inflect.acronym "UUID"
 
diff --git a/config/initializers/omniauth.rb b/config/initializers/omniauth.rb
new file mode 100644
index 00000000..d3c59809
--- /dev/null
+++ b/config/initializers/omniauth.rb
@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+config = Postal::Config.oidc
+if config.enabled?
+  client_options = { identifier: config.identifier, secret: config.secret }
+
+  unless config.discovery?
+    client_options[:authorization_endpoint] = config.authorization_endpoint
+    client_options[:token_endpoint] = config.token_endpoint
+    client_options[:userinfo_endpoint] = config.userinfo_endpoint
+    client_options[:jwks_uri] = config.jwks_uri
+    client_options[:scheme] = config.scheme
+    client_options[:host] = config.host
+    client_options[:port] = config.port
+  end
+
+  Rails.application.config.middleware.use OmniAuth::Builder do
+    provider :openid_connect, name: :oidc,
+                              scope: config.scopes.map(&:to_sym),
+                              uid_field: config.uid_field,
+                              issuer: config.issuer,
+                              discovery: config.discovery?,
+                              client_options: client_options
+  end
+
+  OmniAuth.config.on_failure = proc do |env|
+    SessionsController.action(:oauth_failure).call(env)
+  end
+end
diff --git a/config/routes.rb b/config/routes.rb
index 51026e12..281b0d5a 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -85,6 +85,10 @@
   match "login/reset" => "sessions#begin_password_reset", :via => [:get, :post]
   match "login/reset/:token" => "sessions#finish_password_reset", :via => [:get, :post]
 
+  if Postal::Config.oidc.enabled?
+    get "auth/oidc/callback", to: "sessions#create_from_oidc"
+  end
+
   get "ip" => "sessions#ip"
 
   root "organizations#index"
diff --git a/db/migrate/20240311205229_add_oidc_fields_to_user.rb b/db/migrate/20240311205229_add_oidc_fields_to_user.rb
new file mode 100644
index 00000000..a617222d
--- /dev/null
+++ b/db/migrate/20240311205229_add_oidc_fields_to_user.rb
@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+
+class AddOIDCFieldsToUser < ActiveRecord::Migration[7.0]
+
+  def change
+    add_column :users, :oidc_uid, :string
+    add_column :users, :oidc_issuer, :string
+  end
+
+end
diff --git a/db/schema.rb b/db/schema.rb
index 01d7ef65..9ab08fdb 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.0].define(version: 2024_02_23_141501) do
+ActiveRecord::Schema[7.0].define(version: 2024_03_11_205229) do
   create_table "additional_route_endpoints", id: :integer, charset: "utf8mb4", collation: "utf8mb4_general_ci", force: :cascade do |t|
     t.integer "route_id"
     t.string "endpoint_type"
@@ -330,6 +330,8 @@
     t.string "password_reset_token"
     t.datetime "password_reset_token_valid_until", precision: nil
     t.boolean "admin", default: false
+    t.string "oidc_uid"
+    t.string "oidc_issuer"
     t.index ["email_address"], name: "index_users_on_email_address", length: 8
     t.index ["uuid"], name: "index_users_on_uuid", length: 8
   end
diff --git a/doc/config/environment-variables.md b/doc/config/environment-variables.md
index 37b9a4e0..2d7dab37 100644
--- a/doc/config/environment-variables.md
+++ b/doc/config/environment-variables.md
@@ -97,3 +97,17 @@ This document contains all the environment variables which are available for thi
 | `MIGRATION_WAITER_ENABLED` | Boolean | Wait for all migrations to run before starting a process | false |
 | `MIGRATION_WAITER_ATTEMPTS` | Integer | The number of attempts to try waiting for migrations to complete before start | 120 |
 | `MIGRATION_WAITER_SLEEP_TIME` | Integer | The number of seconds to wait between each migration check | 2 |
+| `OIDC_ENABLED` | Boolean | Enable OIDC authentication | false |
+| `OIDC_NAME` | String | The name of the OIDC provider as shown in the UI | OIDC Provider |
+| `OIDC_ISSUER` | String | The OIDC issuer URL |  |
+| `OIDC_IDENTIFIER` | String | The client ID for OIDC |  |
+| `OIDC_SECRET` | String | The client secret for OIDC |  |
+| `OIDC_SCOPES` | Array of strings | Scopes to request from the OIDC server. | openid |
+| `OIDC_UID_FIELD` | String | The field to use to determine the user's UID | sub |
+| `OIDC_EMAIL_ADDRESS_FIELD` | String | The field to use to determine the user's email address | sub |
+| `OIDC_NAME_FIELD` | String | The field to use to determine the user's name | name |
+| `OIDC_DISCOVERY` | Boolean | Enable discovery to determine endpoints from .well-known/openid-configuration from the Issuer | true |
+| `OIDC_AUTHORIZATION_ENDPOINT` | String | The authorize endpoint on the authorization server (only used when discovery is false) |  |
+| `OIDC_TOKEN_ENDPOINT` | String | The token endpoint on the authorization server (only used when discovery is false) |  |
+| `OIDC_USERINFO_ENDPOINT` | String | The user info endpoint on the authorization server (only used when discovery is false) |  |
+| `OIDC_JWKS_URI` | String | The JWKS endpoint on the authorization server (only used when discovery is false) |  |
diff --git a/doc/config/yaml.yml b/doc/config/yaml.yml
index b9d91d78..9ded5f9d 100644
--- a/doc/config/yaml.yml
+++ b/doc/config/yaml.yml
@@ -219,3 +219,34 @@ migration_waiter:
   attempts: 120
   # The number of seconds to wait between each migration check
   sleep_time: 2
+
+oidc:
+  # Enable OIDC authentication
+  enabled: false
+  # The name of the OIDC provider as shown in the UI
+  name: OIDC Provider
+  # The OIDC issuer URL
+  issuer: 
+  # The client ID for OIDC
+  identifier: 
+  # The client secret for OIDC
+  secret: 
+  # Scopes to request from the OIDC server.
+  scopes:
+    - openid
+  # The field to use to determine the user's UID
+  uid_field: sub
+  # The field to use to determine the user's email address
+  email_address_field: sub
+  # The field to use to determine the user's name
+  name_field: name
+  # Enable discovery to determine endpoints from .well-known/openid-configuration from the Issuer
+  discovery: true
+  # The authorize endpoint on the authorization server (only used when discovery is false)
+  authorization_endpoint: 
+  # The token endpoint on the authorization server (only used when discovery is false)
+  token_endpoint: 
+  # The user info endpoint on the authorization server (only used when discovery is false)
+  userinfo_endpoint: 
+  # The JWKS endpoint on the authorization server (only used when discovery is false)
+  jwks_uri: 
diff --git a/lib/postal/config_schema.rb b/lib/postal/config_schema.rb
index 8e997627..9e2f750f 100644
--- a/lib/postal/config_schema.rb
+++ b/lib/postal/config_schema.rb
@@ -508,6 +508,72 @@ module Postal
         default 2
       end
     end
+
+    group :oidc do
+      boolean :enabled do
+        description "Enable OIDC authentication"
+        default false
+      end
+
+      string :name do
+        description "The name of the OIDC provider as shown in the UI"
+        default "OIDC Provider"
+      end
+
+      string :issuer do
+        description "The OIDC issuer URL"
+      end
+
+      string :identifier do
+        description "The client ID for OIDC"
+      end
+
+      string :secret do
+        description "The client secret for OIDC"
+      end
+
+      string :scopes do
+        description "Scopes to request from the OIDC server."
+        array
+        default "openid"
+      end
+
+      string :uid_field do
+        description "The field to use to determine the user's UID"
+        default "sub"
+      end
+
+      string :email_address_field do
+        description "The field to use to determine the user's email address"
+        default "sub"
+      end
+
+      string :name_field do
+        description "The field to use to determine the user's name"
+        default "name"
+      end
+
+      boolean :discovery do
+        description "Enable discovery to determine endpoints from .well-known/openid-configuration from the Issuer"
+        default true
+      end
+
+      string :authorization_endpoint do
+        description "The authorize endpoint on the authorization server (only used when discovery is false)"
+      end
+
+      string :token_endpoint do
+        description "The token endpoint on the authorization server (only used when discovery is false)"
+      end
+
+      string :userinfo_endpoint do
+        description "The user info endpoint on the authorization server (only used when discovery is false)"
+      end
+
+      string :jwks_uri do
+        description "The JWKS endpoint on the authorization server (only used when discovery is false)"
+      end
+    end
   end
 
   class << self
diff --git a/lib/postal/yaml_config_exporter.rb b/lib/postal/yaml_config_exporter.rb
index f7c2ab94..d85cfb71 100644
--- a/lib/postal/yaml_config_exporter.rb
+++ b/lib/postal/yaml_config_exporter.rb
@@ -19,7 +19,7 @@ def export
               contents << "  #{name}: []"
             else
               contents << "  #{name}:"
-              attr.default.each do |d|
+              attr.transform(attr.default).each do |d|
                 contents << "    - #{d}"
               end
             end
diff --git a/spec/factories/user_factory.rb b/spec/factories/user_factory.rb
index 03373491..5ece8cf3 100644
--- a/spec/factories/user_factory.rb
+++ b/spec/factories/user_factory.rb
@@ -5,19 +5,21 @@
 # Table name: users
 #
 #  id                               :integer          not null, primary key
-#  uuid                             :string(255)
+#  admin                            :boolean          default(FALSE)
+#  email_address                    :string(255)
+#  email_verification_token         :string(255)
+#  email_verified_at                :datetime
 #  first_name                       :string(255)
 #  last_name                        :string(255)
-#  email_address                    :string(255)
+#  oidc_issuer                      :string(255)
+#  oidc_uid                         :string(255)
 #  password_digest                  :string(255)
+#  password_reset_token             :string(255)
+#  password_reset_token_valid_until :datetime
 #  time_zone                        :string(255)
-#  email_verification_token         :string(255)
-#  email_verified_at                :datetime
+#  uuid                             :string(255)
 #  created_at                       :datetime
 #  updated_at                       :datetime
-#  password_reset_token             :string(255)
-#  password_reset_token_valid_until :datetime
-#  admin                            :boolean          default(FALSE)
 #
 # Indexes
 #
diff --git a/spec/models/user/authentication_spec.rb b/spec/models/user/authentication_spec.rb
new file mode 100644
index 00000000..f8dca533
--- /dev/null
+++ b/spec/models/user/authentication_spec.rb
@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+RSpec.describe User do
+  describe ".authenticate" do
+    it "does not authenticate users with invalid emails" do
+      expect { User.authenticate("nothing@nothing.com", "hello") }.to raise_error(Postal::Errors::AuthenticationError) do |e|
+        expect(e.error).to eq "InvalidEmailAddress"
+      end
+    end
+
+    it "does not authenticate users with invalid passwords" do
+      user = create(:user)
+      expect { User.authenticate(user.email_address, "hello") }.to raise_error(Postal::Errors::AuthenticationError) do |e|
+        expect(e.error).to eq "InvalidPassword"
+      end
+    end
+
+    it "authenticates valid users" do
+      user = create(:user)
+      auth_user = nil
+      expect { auth_user = User.authenticate(user.email_address, "passw0rd") }.to_not raise_error
+      expect(auth_user).to eq user
+    end
+  end
+end
diff --git a/spec/models/user/oidc_spec.rb b/spec/models/user/oidc_spec.rb
new file mode 100644
index 00000000..a03668cb
--- /dev/null
+++ b/spec/models/user/oidc_spec.rb
@@ -0,0 +1,115 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+RSpec.describe User do
+  let(:user) { build(:user) }
+
+  describe "#oidc?" do
+    it "returns true if the user has an OIDC UID" do
+      user.oidc_uid = "123"
+      expect(user.oidc?).to be true
+    end
+
+    it "returns false if the user does not have an OIDC UID" do
+      user.oidc_uid = nil
+      expect(user.oidc?).to be false
+    end
+  end
+
+  describe ".find_from_oidc" do
+    let(:issuer) { "https://identity.example.com" }
+
+    before do
+      allow(Postal::Config.oidc).to receive(:enabled?).and_return(true)
+      allow(Postal::Config.oidc).to receive(:issuer).and_return(issuer)
+      allow(Postal::Config.oidc).to receive(:email_address_field).and_return("email")
+    end
+
+    let(:uid) { "abcdef" }
+    let(:oidc_name) { "John Smith" }
+    let(:oidc_email) { "test@example.com" }
+
+    let(:auth) { { "sub" => uid, "email" => oidc_email, "name" => oidc_name } }
+    let(:logger) { TestLogger.new }
+
+    subject(:result) { described_class.find_from_oidc(auth, logger: logger) }
+
+    context "when there is a user that matchers the UID and issuer" do
+      before do
+        @existing_user = create(:user, oidc_uid: uid, oidc_issuer: issuer, first_name: "mary",
+                                       last_name: "apples", email_address: "mary@apples.com")
+      end
+
+      it "returns that user" do
+        expect(result).to eq @existing_user
+      end
+
+      it "updates the name and email address" do
+        result
+        @existing_user.reload
+        expect(@existing_user.first_name).to eq "John"
+        expect(@existing_user.last_name).to eq "Smith"
+        expect(@existing_user.email_address).to eq "test@example.com"
+      end
+
+      it "logs" do
+        result
+        expect(logger).to have_logged(/found user with UID abcdef/i)
+      end
+    end
+
+    context "when there is no user which matches the UID and issuer" do
+      context "when there is a user which matches the email address without an OIDC UID" do
+        before do
+          @existing_user = create(:user, first_name: "mary",
+                                         last_name: "apples", email_address: "test@example.com")
+        end
+
+        it "returns that user" do
+          expect(result).to eq @existing_user
+        end
+
+        it "adds the UID and issuer to the user" do
+          result
+          @existing_user.reload
+          expect(@existing_user.oidc_uid).to eq uid
+          expect(@existing_user.oidc_issuer).to eq issuer
+        end
+
+        it "updates the name if changed" do
+          result
+          @existing_user.reload
+          expect(@existing_user.first_name).to eq "John"
+          expect(@existing_user.last_name).to eq "Smith"
+        end
+
+        it "removes the password" do
+          @existing_user.password = "password"
+          @existing_user.save!
+          result
+          @existing_user.reload
+          expect(@existing_user.password_digest).to be_nil
+        end
+
+        it "logs" do
+          result
+          expect(logger).to have_logged(/no user with UID abcdef/)
+          expect(logger).to have_logged(/found user with e-mail address test@example.com/)
+        end
+      end
+
+      context "when there is no user which matches the email address" do
+        it "returns nil" do
+          expect(result).to be_nil
+        end
+
+        it "logs" do
+          result
+          expect(logger).to have_logged(/no user with UID abcdef/)
+          expect(logger).to have_logged(/no user with e-mail address/)
+        end
+      end
+    end
+  end
+end
diff --git a/spec/models/user_spec.rb b/spec/models/user_spec.rb
index 023ef71d..efbca329 100644
--- a/spec/models/user_spec.rb
+++ b/spec/models/user_spec.rb
@@ -11,6 +11,8 @@
 #  email_verified_at                :datetime
 #  first_name                       :string(255)
 #  last_name                        :string(255)
+#  oidc_issuer                      :string(255)
+#  oidc_uid                         :string(255)
 #  password_digest                  :string(255)
 #  password_reset_token             :string(255)
 #  password_reset_token_valid_until :datetime
@@ -27,34 +29,105 @@
 require "rails_helper"
 
 describe User do
-  context "model" do
-    subject(:user) { create(:user) }
+  subject(:user) { build(:user) }
+
+  describe "validations" do
+    it { is_expected.to validate_presence_of(:first_name) }
+    it { is_expected.to validate_presence_of(:last_name) }
+    it { is_expected.to validate_presence_of(:email_address) }
+    it { is_expected.to validate_presence_of(:password) }
+    it { is_expected.to validate_uniqueness_of(:email_address).case_insensitive }
+    it { is_expected.to allow_value("test@example.com").for(:email_address) }
+    it { is_expected.to allow_value("test@example.co.uk").for(:email_address) }
+    it { is_expected.to allow_value("test+tagged@example.co.uk").for(:email_address) }
+    it { is_expected.to allow_value("test+tagged@EXAMPLE.COM").for(:email_address) }
+    it { is_expected.to_not allow_value("test+tagged").for(:email_address) }
+    it { is_expected.to_not allow_value("test.com").for(:email_address) }
+
+    it "does not require a password when OIDC is enabled" do
+      allow(Postal::Config.oidc).to receive(:enabled?).and_return(true)
+      user.password = nil
+      expect(user.save).to be true
+    end
+  end
+
+  describe "relationships" do
+    it { is_expected.to have_many(:organization_users) }
+    it { is_expected.to have_many(:organizations) }
+  end
+
+  describe "creation" do
+    before { user.save }
 
     it "should have a UUID" do
       expect(user.uuid).to be_a String
       expect(user.uuid.length).to eq 36
     end
+
+    it "has a default timezone" do
+      expect(user.time_zone).to eq "UTC"
+    end
   end
 
-  context ".authenticate" do
-    it "should not authenticate users with invalid emails" do
-      expect { User.authenticate("nothing@nothing.com", "hello") }.to raise_error(Postal::Errors::AuthenticationError) do |e|
-        expect(e.error).to eq "InvalidEmailAddress"
+  describe "#organizations_scope" do
+    context "when the user is an admin" do
+      it "returns a scope of all organizations" do
+        user.admin = true
+        scope = user.organizations_scope
+        expect(scope).to eq Organization.present
       end
     end
 
-    it "should not authenticate users with invalid passwords" do
-      user = create(:user)
-      expect { User.authenticate(user.email_address, "hello") }.to raise_error(Postal::Errors::AuthenticationError) do |e|
-        expect(e.error).to eq "InvalidPassword"
+    context "when the user not an admin" do
+      it "returns a scope including only orgs the user is associated with" do
+        user.admin = false
+        user.organizations << create(:organization)
+        scope = user.organizations_scope
+        expect(scope).to eq user.organizations.present
       end
     end
+  end
+
+  describe "#name" do
+    it "returns the name" do
+      user.first_name = "John"
+      user.last_name = "Doe"
+      expect(user.name).to eq "John Doe"
+    end
+  end
+
+  describe "#password?" do
+    it "returns true if the user has a password" do
+      user.password = "password"
+      expect(user.password?).to be true
+    end
+
+    it "returns false if the user does not have a password" do
+      user.password = nil
+      expect(user.password?).to be false
+    end
+  end
+
+  describe "#to_param" do
+    it "returns the UUID" do
+      user.uuid = "123"
+      expect(user.to_param).to eq "123"
+    end
+  end
+
+  describe "#email_tag" do
+    it "returns the name and email address" do
+      user.first_name = "John"
+      user.last_name = "Doe"
+      user.email_address = "john@example.com"
+      expect(user.email_tag).to eq "John Doe <john@example.com>"
+    end
+  end
 
-    it "should authenticate valid users" do
+  describe ".[]" do
+    it "should find a user by email address" do
       user = create(:user)
-      auth_user = nil
-      expect { auth_user = User.authenticate(user.email_address, "passw0rd") }.to_not raise_error
-      expect(auth_user).to eq user
+      expect(User[user.email_address]).to eq user
     end
   end
 end