OST Lock Error when opening the Outlook Desktop Client

Steps

1. Fully close Outlook + all background processes

Even if Outlook looks closed, it often leaves background processes running that keep the OST file locked.

Steps

1. Press CTRL + SHIFT + ESC à open Task Manager
2. Go to Processes
3. End these if present:
   • Outlook.exe
   • Microsoft Lync / Skype
   • UCMAPI.exe
   • Communicator
   • WebEx
   • Teams
   • Microsoft Word

These are confirmed causes of OST file locking.

Then try reopening Outlook again. 

2. Make sure no other app is touching the OST file

Sync tools sometimes lock OST files temporarily.

Check if any of these apps are running:

• OneDrive syncing your Outlook folder
• Antivirus doing an active email scan
• Backup utilities scanning AppData

Close or pause them, then relaunch Outlook.

Antivirus and file‑scanning tools can indeed lock OST files.

3. Rename the OST file (forces Outlook to rebuild it)

This is safe if your mailbox is Exchange, Microsoft 365, or IMAP.  Please know outlook will recreate the OST automatically.

Steps

1. Close Outlook completely
2. Navigate to: %localappdata%\Microsoft\Outlook
3. Find your OST file  (Match the file name to the error message you received.)
4. Rename it (e.g., filename.ost.bak)
5. Open Outlook à It will generate a fresh OST

4. Disable and re‑enable Cached Exchange Mode (quick unlock trick)

1. Close Outlook
2. Go to Control Panel à Mail à Email Accounts
3. Select your account à Change
4. Uncheck Cached Exchange Mode à Apply à close
5. Reopen Outlook
6. Close Outlook again
7. Re‑enable Cached Exchange Mode

5. Run SCANPST (Inbox Repair Tool)

If the OST has minor corruption, repairing it can unlock it.

SCANPST can repair OST files when Outlook cannot access them. 

Support: Repair Outlook Data files (.pst and .ost)