diff --git a/lib/realtime/realtime.ts b/lib/realtime/realtime.ts
index 112035f7d9..9f8948c42e 100644
--- a/lib/realtime/realtime.ts
+++ b/lib/realtime/realtime.ts
@@ -72,7 +72,7 @@ export interface RealtimeNoteData {
   updatetime: number
 
   // type: ot.EditorSocketIOServer
-  server: any
+  server: ot.EditorSocketIOServer
 
   authors: Record<string, RealtimeAuthorData>
   authorship: Authorship[]
@@ -714,7 +714,7 @@ export function ifMayEdit(socket: SocketIO.Socket, callback: (canEdit: boolean)
 }
 
 // TODO: test it
-function operationCallback(socket: SocketIO.Socket, operation: any) {
+function operationCallback(socket: SocketIO.Socket, operation: ot.Operation[]) {
   const noteId = socket.noteId
   if (!noteId || !notes[noteId]) return
   const note = notes[noteId]
diff --git a/tsconfig.json b/tsconfig.json
index bbc91998dc..85772bade4 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -17,7 +17,8 @@
 
     "typeRoots": ["./typings", "./node_modules/@types"],
     "paths": {
-      "@hackmd/diff-match-patch": ["./typings/diff-match-patch/"]
+      "@hackmd/diff-match-patch": ["./typings/diff-match-patch/"],
+      "ot": ["./typings/ot/"]
     }
   },
   "include": [
diff --git a/typings/ot/index.d.ts b/typings/ot/index.d.ts
new file mode 100644
index 0000000000..e86de954d3
--- /dev/null
+++ b/typings/ot/index.d.ts
@@ -0,0 +1,62 @@
+import SocketIO from "socket.io";
+import EventEmitter from "events";
+
+declare module 'ot' {
+
+  export const version: number
+
+  export type Operation = number | string
+
+  export class TextOperation {
+    static isRetain(op: Operation): boolean
+
+    static isInsert(op: Operation): boolean
+
+    static isDelete(op: Operation): boolean
+  }
+
+  export class EditorSocketIOServer extends EventEmitter {
+
+    public isDirty: boolean
+    public document: string
+    public debug: boolean
+    public operations: Operation[]
+
+    constructor(
+      document: string,
+      operations: Operation[],
+      docId: string,
+      mayWrite: (
+        socket: SocketIO.Socket,
+        callback: (canEdit: boolean) => void) => void,
+      operationCallback: (
+        socket: SocketIO.Socket,
+        operation: Operation[]) => void,
+      operationEventCallback?: any)
+
+    setLogger(logger): void
+
+    debugLog(mes: string): void
+
+    setDocumentMaxLength(number): void
+
+    addClient(socket: SocketIO.Socket): void
+
+    onOperation(socket: SocketIO.Server, revision, operation, selection)
+
+    onGetOperations(socket, base, head)
+
+    updateSelection(socket, selection)
+
+    setName(socket, name)
+
+    setColor(socket, color)
+
+    getClient(clientId)
+
+    onDisconnect(socket)
+  }
+
+}
+
+