diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index bc49d9c2..876da978 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -41,7 +41,7 @@ jobs:
         env:
           USE_BAZEL_VERSION: ${{ matrix.bazel_version }}
           USE_BZLMOD: ${{ matrix.bzlmod }}
-        run: tests/scripts/run_tests.sh
+        run: tests/scripts/run_tests.sh -O
   toolchain_test:
     strategy:
       fail-fast: false
@@ -123,7 +123,7 @@ jobs:
       - name: Test
         env:
           USE_BZLMOD: ${{ matrix.bzlmod }}
-        run: tests/scripts/run_tests.sh -t @llvm_toolchain_with_absolute_paths//:cc-toolchain-x86_64-linux
+        run: tests/scripts/run_tests.sh -t @llvm_toolchain_with_absolute_paths//:cc-toolchain-x86_64-linux -O
   sys_paths_test:
     strategy:
       fail-fast: false
@@ -134,6 +134,7 @@ jobs:
       - uses: actions/checkout@v5
       - run: tests/scripts/ubuntu_install_libtinfo.sh
       - name: Download and Extract LLVM distribution
+        # The downloaded version here must match version specified as '-v' arg to run_tests.sh below.
         env:
           release: llvmorg-16.0.0
           archive: clang+llvm-16.0.0-x86_64-linux-gnu-ubuntu-18.04
@@ -143,4 +144,4 @@ jobs:
       - name: Test
         env:
           USE_BZLMOD: ${{ matrix.bzlmod }}
-        run: tests/scripts/run_tests.sh -t @llvm_toolchain_with_system_llvm//:cc-toolchain-x86_64-linux
+        run: tests/scripts/run_tests.sh -t @llvm_toolchain_with_system_llvm//:cc-toolchain-x86_64-linux -v 16.0.0
diff --git a/MODULE.bazel b/MODULE.bazel
index fa99a1da..bc2fafbe 100644
--- a/MODULE.bazel
+++ b/MODULE.bazel
@@ -20,10 +20,10 @@ module(
 )
 
 bazel_dep(name = "bazel_features", version = "1.38.0")
-bazel_dep(name = "bazel_skylib", version = "1.5.0")
-bazel_dep(name = "aspect_bazel_lib", version = "2.0.0")
-bazel_dep(name = "rules_cc", version = "0.2.2")
-bazel_dep(name = "platforms", version = "0.0.8")
+bazel_dep(name = "bazel_skylib", version = "1.8.2")
+bazel_dep(name = "aspect_bazel_lib", version = "2.19.3")
+bazel_dep(name = "rules_cc", version = "0.2.8")
+bazel_dep(name = "platforms", version = "1.0.0")
 bazel_dep(name = "helly25_bzl", version = "0.3.1")
 
 bazel_dep(name = "tar.bzl", version = "0.6.0")
diff --git a/tests/BUILD.bazel b/tests/BUILD.bazel
index 2589ac76..d34b1308 100644
--- a/tests/BUILD.bazel
+++ b/tests/BUILD.bazel
@@ -81,7 +81,7 @@ build_test(
 dwp_file(
     name = "stdlib.dwp",
     src = ":stdlib_bin",
-    # NOTE: we should eventually we able to drop this; see #109.
+    # NOTE: we should eventually be able to drop this; see #109.
     override_compilation_mode = "dbg",
     target_compatible_with = [
         "@platforms//os:linux",
@@ -101,6 +101,7 @@ cc_test(
     srcs = ["omp_test.c"],
     copts = ["-fopenmp"],
     linkopts = ["-fopenmp"],
+    tags = ["manual"],
     deps = ["@llvm_toolchain//:omp"],
 )
 
@@ -114,9 +115,19 @@ cc_test(
     srcs = ["omp_test.cc"],
     copts = ["-fopenmp"],
     linkopts = ["-fopenmp"],
+    tags = ["manual"],
     deps = ["@llvm_toolchain//:omp"],
 )
 
+test_suite(
+    name = "omp_tests",
+    tags = ["manual"],
+    tests = [
+        ":omp_test",
+        ":omp_test_cc",
+    ],
+)
+
 sh_test(
     name = "file_dependency_test",
     srcs = ["file_dependency_test.sh"],
diff --git a/tests/MODULE.bazel b/tests/MODULE.bazel
index 711f279f..ab7af41f 100644
--- a/tests/MODULE.bazel
+++ b/tests/MODULE.bazel
@@ -22,7 +22,7 @@ local_path_override(
 
 bazel_dep(name = "bazel_skylib", version = "1.8.2")
 bazel_dep(name = "platforms", version = "1.0.0")
-bazel_dep(name = "rules_cc", version = "0.0.9")
+bazel_dep(name = "rules_cc", version = "0.2.8")
 bazel_dep(name = "rules_go", version = "0.50.1", repo_name = "io_bazel_rules_go")
 bazel_dep(name = "rules_rust", version = "0.67.0")
 bazel_dep(name = "rules_foreign_cc", version = "0.15.0")
@@ -63,10 +63,13 @@ llvm = use_extension("@toolchains_llvm//toolchain/extensions:llvm.bzl", "llvm")
 # When updating this version, also update the versions associated with
 # llvm_toolchain below, sys_paths_test in the workflows file, and xcompile_test
 # through the `llvm_toolchain_with_sysroot` toolchain.
+# We use C++17 and the first LLVM version with full support is 16.0.0.
+# We also use C++20 which has reasonable wide support starting with LLVM 17.0.0.
+# MacOS X86 does not exist for LLVM 17 or 18, so we allow 19 as well.
+# We also allow to override this with a environment LLVM_VERSION for testing.
 LLVM_VERSIONS = {
-    "": "16.0.0",
-    "darwin-aarch64": "16.0.5",
-    "darwin-x86_64": "15.0.7",
+    "": "getenv(LLVM_VERSION,latest:>=17.0.0,<20)",
+    "darwin-x86_64": "15.0.7",  # Verify this works as opposed to using one version.
 }
 
 llvm.toolchain(
@@ -147,9 +150,10 @@ llvm.toolchain_root(
 use_repo(llvm, "llvm_toolchain_with_absolute_paths")
 
 # Toolchain example with system LLVM; tested in GitHub CI.
+# The llvm_version must match the version specified in .github/workflows/tests.yml: sys_paths_test
 llvm.toolchain(
     name = "llvm_toolchain_with_system_llvm",
-    llvm_versions = LLVM_VERSIONS,
+    llvm_versions = {"": "16.0.0"},
 )
 
 # For this toolchain to work, the LLVM distribution archive would need to be unpacked here.
diff --git a/tests/WORKSPACE b/tests/WORKSPACE
index af5f1500..dd2d2bdb 100644
--- a/tests/WORKSPACE
+++ b/tests/WORKSPACE
@@ -59,10 +59,13 @@ load("@toolchains_llvm//toolchain:rules.bzl", "llvm_toolchain")
 # When updating this version, also update the versions associated with
 # llvm_toolchain below, sys_paths_test in the workflows file, and xcompile_test
 # through the `llvm_toolchain_with_sysroot` toolchain.
+# We use C++17 and the first LLVM version with full suppor is 16.0.0.
+# We also use C++20 which has reasonable wide support starting with LLVM 17.0.0.
+# MacOS X86 does not exist for LLVM 17 or 18, so we allow 19 as well.
+# We also allow to override this with a environment LLVM_VERSION for testing.
 LLVM_VERSIONS = {
-    "": "16.0.0",
-    "darwin-aarch64": "16.0.5",
-    "darwin-x86_64": "15.0.7",
+    "": "getenv(LLVM_VERSION,latest:>=17.0.0,<20)",
+    "darwin-x86_64": "15.0.7",  # Verify this works as opposed to using one version.
 }
 
 llvm_toolchain(
@@ -134,9 +137,10 @@ llvm_toolchain(
 )
 
 ## Toolchain example with system LLVM; tested in GitHub CI.
+# The llvm_version must match the version specified in .github/workflows/tests.yml: sys_paths_test
 llvm_toolchain(
     name = "llvm_toolchain_with_system_llvm",
-    llvm_versions = LLVM_VERSIONS,
+    llvm_version = "getenv(LLVM_VERSION)",
     # For this toolchain to work, the LLVM distribution archive would need to be unpacked here.
     toolchain_roots = {"": "/opt/llvm-16"},
 )
diff --git a/tests/scripts/archlinux_test.sh b/tests/scripts/archlinux_test.sh
index dd3d4bf1..eb244f4e 100755
--- a/tests/scripts/archlinux_test.sh
+++ b/tests/scripts/archlinux_test.sh
@@ -32,6 +32,6 @@ set -exuo pipefail
 
 # Run tests
 cd /src
-tests/scripts/run_tests.sh -t ${toolchain}
+tests/scripts/run_tests.sh -O -t ${toolchain}
 """
 done
diff --git a/tests/scripts/debian_test.sh b/tests/scripts/debian_test.sh
index c0de9666..0fb2858e 100755
--- a/tests/scripts/debian_test.sh
+++ b/tests/scripts/debian_test.sh
@@ -36,6 +36,6 @@ apt-get -qq -y install curl libtinfo5 libxml2 zlib1g-dev >/dev/null
 
 # Run tests
 cd /src
-tests/scripts/run_tests.sh
+tests/scripts/run_tests.sh -O
 """
 done
diff --git a/tests/scripts/fedora_test.sh b/tests/scripts/fedora_test.sh
index 1cc4cce5..20e31994 100755
--- a/tests/scripts/fedora_test.sh
+++ b/tests/scripts/fedora_test.sh
@@ -32,6 +32,6 @@ dnf install -qy glibc-headers ncurses-compat-libs
 
 # Run tests
 cd /src
-tests/scripts/run_tests.sh
+tests/scripts/run_tests.sh -O
 """
 done
diff --git a/tests/scripts/linux_sysroot_test.sh b/tests/scripts/linux_sysroot_test.sh
index a2935c24..3cbd0d6b 100755
--- a/tests/scripts/linux_sysroot_test.sh
+++ b/tests/scripts/linux_sysroot_test.sh
@@ -19,6 +19,8 @@ images=(
   "ubuntu:22.04"
 )
 
+LLVM_VERSION="first:>=15.0.0,<17"
+
 git_root=$(git rev-parse --show-toplevel)
 readonly git_root
 
@@ -36,6 +38,6 @@ apt-get -qq -y install curl libtinfo5 libxml2 zlib1g-dev >/dev/null
 
 # Run tests
 cd /src
-tests/scripts/run_tests.sh -t '@llvm_toolchain_with_sysroot//:cc-toolchain-x86_64-linux'
+tests/scripts/run_tests.sh -t '@llvm_toolchain_with_sysroot//:cc-toolchain-x86_64-linux' -v '${LLVM_VERSION}'
 """
 done
diff --git a/tests/scripts/run_tests.sh b/tests/scripts/run_tests.sh
index abca1b20..48c50742 100755
--- a/tests/scripts/run_tests.sh
+++ b/tests/scripts/run_tests.sh
@@ -16,18 +16,28 @@
 set -euo pipefail
 
 toolchain_name=""
-disable_wasm_tests=""
+enable_omp_targets="1"
+enable_wasm_tests="1"
+LLVM_VERSION=""
 
-while getopts "t:hW" opt; do
+while getopts "hOt:v:W" opt; do
   case "${opt}" in
-  "t") toolchain_name="${OPTARG}" ;;
   "h")
     echo "Usage:"
     echo "-t - Toolchain name to use for testing; default is llvm_toolchain"
     exit 2
     ;;
+  "O")
+    enable_omp_targets=""
+    ;;
+  "t")
+    toolchain_name="${OPTARG}"
+    ;;
+  "v")
+    LLVM_VERSION="${OPTARG}"
+    ;;
   "W")
-    disable_wasm_tests="yes"
+    enable_wasm_tests=""
     ;;
   *)
     echo "invalid option: -${OPTARG}"
@@ -56,10 +66,21 @@ targets=(
 # :test_cxx_standard_is_20 builds with a version of the default toolchain, if
 # we're trying to build with a different toolchain then it's likely the default
 # toolchain won't work so :test_cxx_standard_is_20 won't build.
-if [[ -z ${toolchain_name} ]]; then
+if [[ -z "${toolchain_name}" ]]; then
   targets+=("//:test_cxx_standard_is_20")
 fi
 
+if [[ -n "${enable_omp_targets}" ]]; then
+  targets+=("//:omp_tests")
+fi
+
+if [[ -n "${LLVM_VERSION}" ]]; then
+  echo "LLVM_VERSION=${LLVM_VERSION}"
+  common_test_args+=(
+    "--repo_env=LLVM_VERSION=${LLVM_VERSION}"
+  )
+fi
+
 "${bazel}" ${TEST_MIGRATION:+"--strict"} --bazelrc=/dev/null test \
   "${common_test_args[@]}" "${test_args[@]}" "${targets[@]}"
 
@@ -72,7 +93,7 @@ fi
 # to run out of disk space.
 #
 # Mitigate this by expunging the workspace before trying to build Wasm targets.
-if [[ -z ${toolchain_name} && -z ${disable_wasm_tests} ]]; then
+if [[ -z "${toolchain_name}" ]] && [[ -n "${enable_wasm_tests}" ]]; then
   # Redefine `test_args` without `--linkopt=-Wl,-v`, which breaks `wasm-ld`.
   #
   # https://github.com/llvm/llvm-project/issues/112836
diff --git a/tests/scripts/suse_leap_test.sh b/tests/scripts/suse_leap_test.sh
index 7ba31974..3e3308d5 100755
--- a/tests/scripts/suse_leap_test.sh
+++ b/tests/scripts/suse_leap_test.sh
@@ -38,6 +38,6 @@ zypper -n install curl gcc libc++1
 
 # Run tests
 cd /src
-tests/scripts/run_tests.sh -t ${toolchain}
+tests/scripts/run_tests.sh -O -t ${toolchain}
 """
 done
diff --git a/tests/scripts/suse_tumbleweed_test.sh b/tests/scripts/suse_tumbleweed_test.sh
index e5d6cd82..07db78a7 100755
--- a/tests/scripts/suse_tumbleweed_test.sh
+++ b/tests/scripts/suse_tumbleweed_test.sh
@@ -38,6 +38,6 @@ zypper -n install curl gcc libc++1
 
 # Run tests
 cd /src
-tests/scripts/run_tests.sh -t ${toolchain}
+tests/scripts/run_tests.sh -O -t ${toolchain}
 """
 done
diff --git a/tests/scripts/ubuntu_20_04_test.sh b/tests/scripts/ubuntu_20_04_test.sh
index 38e6d20e..a8510a14 100755
--- a/tests/scripts/ubuntu_20_04_test.sh
+++ b/tests/scripts/ubuntu_20_04_test.sh
@@ -19,6 +19,8 @@ images=(
   "ubuntu:20.04"
 )
 
+LLVM_VERSION="first:>=15.0.0,<17"
+
 git_root=$(git rev-parse --show-toplevel)
 readonly git_root
 
@@ -40,6 +42,6 @@ disable_wasm_tests='-W'
 
 # Run tests
 cd /src
-tests/scripts/run_tests.sh \${disable_wasm_tests}
+tests/scripts/run_tests.sh -v '${LLVM_VERSION}' \${disable_wasm_tests}
 """
 done
diff --git a/tests/scripts/ubuntu_22_04_test.sh b/tests/scripts/ubuntu_22_04_test.sh
index f5341fd8..2e5ff013 100755
--- a/tests/scripts/ubuntu_22_04_test.sh
+++ b/tests/scripts/ubuntu_22_04_test.sh
@@ -36,6 +36,6 @@ apt-get -qq -y install curl libtinfo5 libxml2 zlib1g-dev >/dev/null
 
 # Run tests
 cd /src
-tests/scripts/run_tests.sh
+tests/scripts/run_tests.sh -O
 """
 done
diff --git a/toolchain/internal/configure.bzl b/toolchain/internal/configure.bzl
index 279e1094..45d20556 100644
--- a/toolchain/internal/configure.bzl
+++ b/toolchain/internal/configure.bzl
@@ -38,7 +38,10 @@ load(
     _supported_targets = "SUPPORTED_TARGETS",
     _toolchain_tools = "toolchain_tools",
 )
-load("//toolchain/internal:llvm_distributions.bzl", "is_requirement", "required_llvm_release_name_rctx")
+load(
+    "//toolchain/internal:llvm_distributions.bzl",
+    _required_llvm_version_rctx = "required_llvm_version_rctx",
+)
 load(
     "//toolchain/internal:sysroot.bzl",
     _default_sysroot_path = "default_sysroot_path",
@@ -81,16 +84,7 @@ def llvm_config_impl(rctx):
 
     if not toolchain_root:
         fail("LLVM toolchain root missing for ({}, {})".format(os, arch))
-    _, llvm_version = _exec_os_arch_dict_value(rctx, "llvm_versions")
-    if is_requirement(llvm_version):
-        llvm_version, distribution, error = required_llvm_release_name_rctx(rctx, llvm_version)
-        if error:
-            fail(error)
-        if llvm_version:
-            print("\nINFO: Resolved latest LLVM version to {llvm_version}: {distribution}".format(
-                distribution = distribution,
-                llvm_version = llvm_version,
-            ))  # buildifier: disable=print
+    llvm_version = _required_llvm_version_rctx(rctx)
     if not llvm_version:
         # LLVM version missing for (os, arch)
         _empty_repository(rctx)
diff --git a/toolchain/internal/llvm_distributions.bzl b/toolchain/internal/llvm_distributions.bzl
index 2721b7ca..6d2f6e79 100644
--- a/toolchain/internal/llvm_distributions.bzl
+++ b/toolchain/internal/llvm_distributions.bzl
@@ -1125,12 +1125,23 @@ def _find_llvm_basename_or_error(llvm_version, all_llvm_distributions, host_info
 
     return basenames[0], None
 
+def _is_requirement(version_or_requirements):
+    """Return whether `version_or_requirements` is likely a requirement (True) or should be a version."""
+    if version_or_requirements.startswith("getenv("):
+        return True
+    for prefix in ["first:", "latest:"]:
+        if version_or_requirements.startswith(prefix) or version_or_requirements == prefix[:-1]:
+            return True
+    return False
+
 def _parse_version_or_requirements(version_or_requirements):
-    if version_or_requirements in ["latest", "first"]:
-        return None
     for prefix in ["latest:", "first:"]:
         if version_or_requirements.startswith(prefix):
             return versions.parse_requirements(version_or_requirements.removeprefix(prefix))
+    if version_or_requirements in ["latest", "first"]:
+        return None
+    if not _is_requirement(version_or_requirements):
+        return None
     fail("ERROR: Invalid version requirements: '{version_or_requirements}'.".format(
         version_or_requirements = version_or_requirements,
     ))
@@ -1159,11 +1170,43 @@ def _required_llvm_release_name(*, version_or_requirements, all_llvm_distributio
             return llvm_version, basenames[0], None
     return None, None, "ERROR: No matching distribution found."
 
-def required_llvm_release_name_rctx(rctx, llvm_version):
+def _resolve_llvm_version_rctx_env(rctx, llvm_version):
+    if llvm_version.startswith("getenv(") and llvm_version.endswith(")"):
+        env_var = llvm_version[len("getenv("):-1]
+        if env_var.find(",") >= 0:
+            env_name, env_default = env_var.split(",", 1)
+        else:
+            env_name = env_var
+            env_default = None
+
+        env_name = env_name.strip(" ")
+        if not env_var:
+            fail("ERROR: Bad getenv parameter '{env_var}'.".format(
+                env_var = env_var,
+            ))
+
+        # We prefer 'repository_ctx.getenv' if it is available (~7.1+) and default
+        # to accessing the environment directly. The latter breaks "hermeticity".
+        if hasattr(rctx, "getenv"):
+            llvm_version = rctx.getenv(env_name, env_default)
+        elif env_name in rctx.os.environ:
+            llvm_version = rctx.os.environ[env_name]
+        else:
+            llvm_version = env_default
+
+        if not llvm_version:
+            fail("ERROR: Empty getenv lookup for '{env_var}'.".format(
+                env_var = env_var,
+            ))
+
+    return llvm_version
+
+def _required_llvm_release_name_rctx(rctx, llvm_version):
+    llvm_version = _resolve_llvm_version_rctx_env(rctx, llvm_version)
     all_llvm_distributions = _get_all_llvm_distributions(
         llvm_distributions = _llvm_distributions,
         extra_llvm_distributions = rctx.attr.extra_llvm_distributions,
-        parsed_llvm_version = _parse_version(llvm_version),
+        parsed_llvm_version = _parse_version(llvm_version) if not _is_requirement(llvm_version) else None,
     )
     return _required_llvm_release_name(
         version_or_requirements = llvm_version,
@@ -1171,12 +1214,19 @@ def required_llvm_release_name_rctx(rctx, llvm_version):
         host_info = host_info(rctx),
     )
 
-def is_requirement(version_or_requirement):
-    """Return whether `version_or_requirement` is likely a requirement (True) or should be a version."""
-    for prefix in ["first:", "latest:"]:
-        if version_or_requirement.startswith(prefix) or version_or_requirement == prefix[:-1]:
-            return True
-    return False
+def required_llvm_version_rctx(rctx):
+    _, llvm_version = exec_os_arch_dict_value(rctx, "llvm_versions")
+    if _is_requirement(llvm_version):
+        llvm_version, distribution, error = _required_llvm_release_name_rctx(rctx, llvm_version)
+        if error:
+            fail(error)
+        if llvm_version:
+            print("\nINFO: Resolved latest LLVM version for {name} to {llvm_version}: {distribution}".format(
+                name = rctx.attr.name,
+                distribution = distribution,
+                llvm_version = llvm_version,
+            ))  # buildifier: disable=print
+    return llvm_version
 
 def _filter_llvm_distributions(*, llvm_version, all_llvm_distributions):
     """Return (distribution: sha) entries from `all_llvm_distributions` that match `llvm_version`."""
@@ -1192,13 +1242,14 @@ def _distribution_urls(rctx):
     all_llvm_distributions = _get_all_llvm_distributions(
         llvm_distributions = _llvm_distributions,
         extra_llvm_distributions = rctx.attr.extra_llvm_distributions,
-        parsed_llvm_version = _parse_version(llvm_version),
+        parsed_llvm_version = _parse_version(llvm_version) if not _is_requirement(llvm_version) else None,
     )
     _, sha256, strip_prefix, _ = _key_attrs(rctx)
 
     if rctx.attr.distribution == "auto":
         rctx_host_info = host_info(rctx)
-        if is_requirement(llvm_version):
+        llvm_version = _resolve_llvm_version_rctx_env(rctx, llvm_version)
+        if _is_requirement(llvm_version):
             llvm_version, basename, error = _required_llvm_release_name(
                 version_or_requirements = llvm_version,
                 all_llvm_distributions = all_llvm_distributions,
diff --git a/toolchain/internal/repo.bzl b/toolchain/internal/repo.bzl
index 934b5373..99e2ee3d 100644
--- a/toolchain/internal/repo.bzl
+++ b/toolchain/internal/repo.bzl
@@ -14,13 +14,13 @@
 load(
     "//toolchain/internal:common.bzl",
     _attr_dict = "attr_dict",
-    _exec_os_arch_dict_value = "exec_os_arch_dict_value",
     _os = "os",
     _supported_os_arch_keys = "supported_os_arch_keys",
 )
 load(
     "//toolchain/internal:llvm_distributions.bzl",
     _download_llvm = "download_llvm",
+    _required_llvm_version_rctx = "required_llvm_version_rctx",
 )
 
 _target_pairs = ", ".join(_supported_os_arch_keys())
@@ -423,8 +423,7 @@ def llvm_repo_impl(rctx):
         rctx.file("BUILD.bazel", executable = False)
         return None
 
-    _, llvm_version = _exec_os_arch_dict_value(rctx, "llvm_versions")
-
+    llvm_version = _required_llvm_version_rctx(rctx)
     major_llvm_version = int(llvm_version.split(".")[0])
 
     rctx.file(