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Summary: In this blog, we will share some easy methods to fix “OST file is in use and cannot be accessed” error in Outlook. There may be any reason, like a large or corrupted PST file, faulty or incompatible add-ins, or an antivirus locking up the OST file. Follow the steps carefully to remove the “Outlook OST file in use and cannot be accessed” error and get your Outlook back to work again.

Imagine you are trying to access your OST file data, but suddenly you come face-to-face with the – OST File in Use Cannot be Accessed error. Frustating right? Don’t worry, you are not alone; various users face this error.

Outlook saves its mailbox data in an .ost file locally on your system. But in certain situations, when one tries to open a username.ost file, but can’t access it, then it can cause an issue. You were not able to open or send your emails.

Reasons for “OST File is in Use and Cannot be Accessed” error

There are a few reasons that you are facing the “Outlook data file ost is in use and cannot be accessed” error. Some of the common reasons for facing the issue include;

  • Outlook is still running in the background even after you close it.
  • Any backup software, antivirus, or sync tool uses the OST file.
  • The OST file is corrupted or damaged.
  • Add-ins or plugins are causing conflicts.
  • Slow internet or sync problems with Exchange/Office 365.
  • Issues with Cached Exchange Mode in Outlook.

How to Fix “OST file is in use and cannot be accessed” Error?

1. Close Outlook Completely

Sometimes, if Outlook continues to run in the background even after closing, you will face the error. Here are the steps to close your Outlook completely.

  1. Open Task Manager
    task-manager
  2. End all processes named OUTLOOK.EXE or Communicator.exe
    end-task
  3. Restart Outlook

2. Restart Your PC

A simple restart clears any background process holding the OST file. After reboot, try launching Outlook again.

3. Disable Cached Exchange Mode Temporarily

If multiple devices or apps are syncing, Cached Exchange Mode can lock the OST.
user-account

  1. Open Control Panel → Mail → Email Accounts
  2. Select your account → Change
    change-settings
  3. Uncheck Use Cached Exchange Mode → Restart Outlook
    cached-mode
  4. If Outlook works fine, re-enable Cached Exchange Mode later

4. Disable Problematic Add-ins

Faulty add-ins often cause the “OST File in Use & Cannot be Accessed” error. Disable them,

  1. In Outlook → go to File > Options > Add-ins
  2. Choose COM Add-ins → Go
    add-ins
  3. Uncheck all, restart Outlook
  4. Enable add-ins one by one to identify the culprit

5. Recreate the OST File

If nothing works, recreate OST file (safe method — no data loss as the mailbox re-syncs from the server):

  1. Close Outlook
  2. Go to C:\Users\YourName\AppData\Local\Microsoft\Outlook
  3. Rename the .ost file (e.g., add .old)
  4. Restart Outlook → a fresh OST will be created

6. Repair the OST File

If the OST is corrupted, repair OST file with the inbuilt Inbox Repair Tool

  1. Locate SCANPST.exe (found in your Office installation folder)
  2. Browse and select your .ost file
  3. Click Start → Repair
  4. Restart Outlook

7. Disable Cache Exchange Mode

This error can be caused by the enabled cache mode in your Exchange Server. Disabling it can help you solve the error. Follow the instructions –

  1. Open the Outlook application and click on the File.
  2. Then go to Account Settings >> click on Account Settings again.
  3. Choose your Exchange account and click Change.
  4. Turn off Use Cached Exchange Mode by removing the check mark next to it, then click Next.
  5. After that, click Finish, then Close.
  6. Close Outlook and open it again to apply the changes.

If you are still facing the OST file in use cannot open error, then you must move to professional help.

Alternative Solution to Fix OST File in Use Error in Outlook

If the OST file you are trying to open is an orphan, then all the above solutions will not work. In that case, you need a professional tool. You can use the BLR OST to PST Converter software. Moreover, it supports damaged, corrupted, or inaccessible OST file without affecting your data. To experience its working, you can try out the demo version of the tool.

download-free-software

Additionally, using this tool, you can also convert OST to PDF, PST, MBOX, TXT, CSV, EML, and many other file formats. Try out the software demo version and analyze how it works.

Conclusion

The error “OST File is in use and cannot be accessed” prevents the user from accessing emails. The error usually happens when Outlook is still running in the background, or the file gets damaged. You can fix it with easy steps like closing Outlook properly, restarting your computer, or turning off add-ins. Follow the steps we mentioned in this post and solve the issue. Hope the given solution helps you to fix the error.

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