Repair Outlook OST File Not Working in Office 365 – Complete Fix (2025)

If your Outlook OST file isn’t syncing properly with Office 365, you’re not alone. Thousands of users face corrupted OST files, sync errors, and missing mailbox data after system updates or unexpected shutdowns. In this 2025 guide, we’ll show you how to repair the Outlook OST file and restore seamless email performance without losing important data.

Why OST Files Get Corrupted or Stop Working

• Sudden Outlook crashes or system shutdowns during sync.
• Large mailbox size exceeding the recommended 5 GB limit.
• Improper termination of Outlook sessions.
• Network interruptions while syncing with Office 365 servers.
• Outdated add-ins or antivirus programs interfering with the OST file.

Step-by-Step Guide to Repair Outlook OST File in Office 365

Step 1: Close Outlook Completely

Before starting any repair, close Outlook and confirm via Task Manager that no background process is running. This prevents further damage to the OST file while it’s being repaired.

Step 2: Locate Your OST File

Default OST file location:

C:\Users\%username%\AppData\Local\Microsoft\Outlook\

You’ll see files ending with “.ost” — these store your cached mailbox data.

Step 3: Use the Inbox Repair Tool (SCANPST.EXE)

Microsoft provides a built-in tool called SCANPST.EXE to scan and repair minor OST corruption.

1. Navigate to: C:\Program Files (x86)\Microsoft Office\root\Office16
2. Run SCANPST.EXE.
3. Browse and select your OST file.
4. Click Start → Repair → Finish.

Step 4: Recreate a Fresh OST File

If the repair fails, rename the old OST file and let Outlook create a new one automatically:

1. Close Outlook.
2. Rename the OST file (e.g., outlook-old.ost).
3. Re-open Outlook → the app will rebuild a fresh cache from the Office 365 server.

Step 5: Run Outlook in Safe Mode to Check Add-Ins

Sometimes faulty add-ins cause sync errors. Run Outlook in Safe Mode:

outlook /safe

If the OST works properly, disable unnecessary add-ins from File → Options → Add-ins.

Step 6: Check Server Connection Status

Navigate to Send/Receive → Work Offline and toggle it. If Outlook reconnects successfully, the OST issue was likely caused by temporary disconnection from Office 365 servers.

Prevent Future OST Corruption

• Keep your Office 365 apps updated via Windows Update.
• Never force close Outlook during synchronization.
• Regularly archive old emails to reduce OST size.
• Whitelist Outlook processes in your antivirus to prevent file locks.

Advanced Repair Option for Severe Corruption

If the built-in tools fail, you can export the mailbox to a PST file and import it again. Follow these steps:

1. Open Outlook → File → Open & Export → Import/Export.
2. Select “Export to a file” → “Outlook Data File (.pst)”.
3. After exporting, remove and re-add the account to generate a new OST file.
4. Import your PST back into the new profile.

Frequently Asked Questions

Why does my OST file keep corrupting?

Large mailbox sizes, unstable network connections, and abrupt shutdowns are the leading causes of OST corruption.

Can I delete my OST file without losing data?

Yes — Outlook will re-sync mailbox data from the server once you relaunch the app, as long as you use an Exchange or Office 365 account.

Does FixTechGuide offer help for OST repair?

Yes ✅ You can connect with our experts on WhatsApp for quick remote support and step-by-step troubleshooting guidance.

Conclusion

A corrupted OST file can pause your business communication, but with these repair steps you can restore your Outlook in minutes. Follow each method carefully, keep your account synced to Office 365, and maintain regular backups to avoid future data loss.

🔗 Related Articles

• Repair Outlook Data File (OST & PST) – 2025 Guide
• Fix Outlook and Office 365 Sync Issues Easily

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