diff --git a/backend/open_webui/routers/auths.py b/backend/open_webui/routers/auths.py
index 5e054596c..8d7581dd4 100644
--- a/backend/open_webui/routers/auths.py
+++ b/backend/open_webui/routers/auths.py
@@ -479,19 +479,25 @@ async def ldap_auth(
             user = Users.get_user_by_email(email, db=db)
             if not user:
                 try:
-                    role = 'admin' if not Users.has_users(db=db) else request.app.state.config.DEFAULT_USER_ROLE
-
+                    # Insert with default role first to avoid TOCTOU race on
+                    # first-user registration.  Matches signup_handler pattern.
                     user = Auths.insert_new_auth(
                         email=email,
                         password=str(uuid.uuid4()),
                         name=cn,
-                        role=role,
+                        role=request.app.state.config.DEFAULT_USER_ROLE,
                         db=db,
                     )
 
                     if not user:
                         raise HTTPException(500, detail=ERROR_MESSAGES.CREATE_USER_ERROR)
 
+                    # Atomically check if this is the only user *after* the
+                    # insert.  Only the single user present should become admin.
+                    if Users.get_num_users(db=db) == 1:
+                        Users.update_user_role_by_id(user.id, 'admin', db=db)
+                        user = Users.get_user_by_id(user.id, db=db)
+
                     apply_default_group_assignment(
                         request.app.state.config.DEFAULT_GROUP_ID,
                         user.id,
diff --git a/backend/open_webui/utils/oauth.py b/backend/open_webui/utils/oauth.py
index 8aaadc271..100df7a21 100644
--- a/backend/open_webui/utils/oauth.py
+++ b/backend/open_webui/utils/oauth.py
@@ -1109,9 +1109,12 @@ class OAuthManager:
             log.debug('Assigning the only user the admin role')
             return 'admin'
         if not user and user_count == 0:
-            # If there are no users, assign the role "admin", as the first user will be an admin
-            log.debug('Assigning the first user the admin role')
-            return 'admin'
+            # First-user bootstrap: skip role management gating so the
+            # instance can be initialized.  We intentionally return the
+            # default role here (not 'admin') — admin promotion happens
+            # race-safely *after* insert via get_num_users() == 1.
+            log.debug('First user bootstrap: using default role (admin promotion deferred to post-insert)')
+            return auth_manager_config.DEFAULT_USER_ROLE
 
         if auth_manager_config.ENABLE_OAUTH_ROLE_MANAGEMENT:
             log.debug('Running OAUTH Role management')
@@ -1577,6 +1580,16 @@ class OAuthManager:
                         db=db,
                     )
 
+                    if not user:
+                        raise HTTPException(500, detail=ERROR_MESSAGES.CREATE_USER_ERROR)
+
+                    # Atomically check if this is the only user *after* the
+                    # insert to avoid TOCTOU race on first-user registration.
+                    # Matches signup_handler pattern.
+                    if Users.get_num_users(db=db) == 1:
+                        Users.update_user_role_by_id(user.id, 'admin', db=db)
+                        user = Users.get_user_by_id(user.id, db=db)
+
                     if auth_manager_config.WEBHOOK_URL:
                         await post_webhook(
                             WEBUI_NAME,