Email Sync Problems

How to Fix Office Apps Keeping Outlook OST File Open

Bushra Parveen Sep 30, 2026 868 views

Question details

The user is unable to delete or reset their Outlook OST file because other Microsoft Office applications are keeping the file open.

How to Fix Office Apps Keeping Outlook OST File Open

Product: Microsoft Office
Device & OS: not provided
Scenario: Attempting to delete or reset an Outlook OST file to troubleshoot email synchronization issues.
Observed behavior: The system blocks the deletion or modification of the OST file, stating that it is currently in use by another Microsoft Office program such as Excel, Word, or OneNote.

Before you start

Ensure you have saved all your work in your Microsoft Office applications, as you will need to close them completely to release the lock on the OST file.

Solution 1Recommended

Close All Office Applications and Reset the OST File

By terminating all Microsoft 365 and Office processes, you can release the shared components' lock on the OST file to safely delete or reset it.

Microsoft Office apps like Word, Excel, and OneNote share Microsoft 365 or Exchange account components and cached data with Outlook. This shared infrastructure causes these apps to hold the OST file open even if the main Outlook window is closed.

Close Microsoft Outlook

Save any draft emails and exit the Microsoft Outlook application completely.

Exit other Office apps

Close all other active Microsoft Office applications, including Word, Excel, PowerPoint, and OneNote.

End background processes

Right-click the Windows taskbar and select Task Manager (or press Ctrl + Shift + Esc). Look for background Office processes like WINWORD.EXE, EXCEL.EXE, or ONENOTE.EXE. Select them and click End Task.

Delete or rename the OST file

Press Win + R, type %localappdata%\Microsoft\Outlook, and press Enter. Locate your .ost file, right-click it, and select Delete or Rename.

Rebuild the OST file

Open Microsoft Outlook. The application will automatically connect to your mail server and create a fresh OST file with newly synced data.

Close All Office Applications and Reset the OST File

Timing Your Reset: If you are in a corporate environment or have heavy background syncing, perform the reset outside business hours when no background Office applications or automated sync processes are active.

Free Microsoft Office alternative

Switch to WPS Office for a Lightweight and Independent Office Experience

If background processes and shared data locks in Microsoft Office are disrupting your workflow, consider trying WPS Office. It provides a lightweight, highly compatible alternative without the heavy background resource sharing that complicates system file management.

No complicated shared background processes locking your system files.Fully compatible with Microsoft Office formats (.docx, .xlsx, .pptx).Lightweight installation that runs smoothly on all Windows, Mac, and Linux devices.Familiar user interface for a seamless transition from Microsoft Office.

Frequently Asked Questions

Why do apps like Excel or Word need to access the Outlook OST file?

Microsoft 365 applications are deeply integrated. Excel, Word, and OneNote use shared account components and cached data for features like 'Share via Email' or inserting Outlook contacts, which keeps the OST file active.

Can I delete the OST file without closing Word or Excel?

No, Windows will block the deletion because the file is in use. You must close all programs that share the Exchange or Microsoft 365 account data, including background processes.

Where is the Outlook OST file normally located?

By default, the OST file is located in a hidden AppData folder. You can quickly access it by pressing Win + R, typing %localappdata%\Microsoft\Outlook in the Run dialog, and pressing Enter.

What happens when I delete my OST file?

Deleting the OST file removes the local offline cache of your mailbox. When you restart Outlook, it will automatically connect to your Exchange or IMAP server and download a fresh copy of your emails to a brand new OST file.

Outlook & Email Problems