Outlook OST File is in Use and Cannot be Accessed Error

What Causes the OST Access Error?

Microsoft documents no single cause for the message. Two Microsoft Q&A threads from 2018 give the process and cached mode clues. Safe mode tests the add in clue:

• Another process. In one Microsoft Q&A thread from March 2018 the user saw the error after an update restart while Skype for Business was still starting. That user reported that letting Skype for Business finish first avoided the errors. Outlook also reported that only one version can open at a time.
• Damaged Office files. The accepted answer in that thread was a restart followed by an Office repair.
• Add ins. Microsoft documents no add in cause for this message. Safe mode shows whether an add in is involved.
• Cached mode. A second Microsoft Q&A thread from January 2018 listed Cached Exchange Mode off as a test. That poster reported no result.

Which Startup Clue do You See?

Match your clue to a method first:

5 Ways to Fix Outlook OST File is in Use and Cannot be Accessed

Four methods use Windows and Outlook settings. Software handles the fifth:

Method #1: End Outlook Processes in Task Manager (Free)

Another process can use the file when Outlook starts. Six steps release it:

1. Close every Outlook window and save your work.
2. Press Ctrl + Shift + Esc to open Task Manager.
3. Open the Processes tab and find Microsoft Outlook.
4. Select the entry and press End task.
5. Repeat for Skype for Business or Lync when they appear in the list.
6. Restart the computer and open Outlook.

Note: A community answer in the second thread named Communicator, Lync and ucmapi as processes to close. Let Skype for Business finish starting before you open Outlook because the person who asked in the first thread reported that order avoided the errors.

Method #2: Open Outlook in Safe Mode and Disable Add Ins (Free)

Microsoft states in its safe mode article that safe mode disables add ins. Six steps test them:

1. Press Windows + R on the keyboard.
2. Enter outlook.exe /safe and press OK.
3. Choose your profile in the dialog and press OK.
4. Open File then Options and select Add-ins.
5. Choose COM Add-ins in the Manage box and press Go.
6. Clear the boxes and close Outlook before you open it again as usual.

Note: Microsoft's safe mode article suggests the full path to Outlook when Windows cannot find the command and gives the Office version folder as its example. Tick the add ins again one at a time to find the one that causes the error.

Method #3: Repair Office From Apps and Features (Free)

Microsoft describes the first repair option as a check that detects and replaces corrupted files. One Microsoft Q&A thread accepted this repair after a restart as its answer. Five steps start it:

1. Right click Start and open Apps and Features.
2. Find your Office entry in the list and press Modify.
3. Choose Quick Repair and press Repair.
4. Restart the computer when the repair ends.
5. Open Outlook and repeat steps 1 to 3 but choose Online Repair if the error remains.

Note: Microsoft's repair article states that repairing one Office app repairs the full suite. Microsoft describes Online Repair as the option that makes sure everything gets fixed.

Method #4: Switch Off Cached Exchange Mode to Test the Error (Free)

One community answer in the second thread listed this test for the error. According to the Microsoft article on Cached Exchange Mode the mode downloads your mail to an Outlook data file and does not apply to POP or IMAP accounts. Six steps test it:

1. Close Outlook and go to Control Panel.
2. Open Mail (Microsoft Outlook) and press Email Accounts.
3. Select your Exchange account on the Email tab and press Change.
4. Clear Use Cached Exchange Mode under Offline Settings.
5. Press Next and then Finish.
6. Open Outlook and tick the box again once the error is gone.

Note: Microsoft recommends the mode as the default for Exchange accounts and states that the setting is missing when an administrator disables it.

Method #5: Convert the OST File to PST With EmailDoctor OST to PST Converter (Trial Limit Applies)

Product guide says EmailDoctor OST to PST Converter opens corrupt and orphan OST files with no Outlook and no Exchange profile. Use a copy so the original file remains safe.

Open the OST Copy in the Software

Three steps add the file:

1. Open the converter and press Add File on the first screen.
2. Choose Select File in the dialog and use the ... button to browse to the copy.
3. Select Quick scan and press Add. Switch to Advanced when the preview shows no data.

Check the Folders and Save the PST File

Three steps finish the export:

4. Look through the folder list after the scan because bold folders contain data.
5. Press Export and select PST as the output.
6. Select the destination folder with Browse and press Export.

Note: Product guide puts the trial export at 25 items for every folder. Test one folder before the full export.

What do the Results Tell You?

Compare the outcome before the next test:

Frequently Asked Questions

Author’s Verdict

Close the Outlook and Skype processes first because Microsoft Q&A users tied the error to another process. Test safe mode and repair Office next.

Test Cached Exchange Mode on Exchange accounts only. Choose converter software if the OST file never opens.

Backup the OST file before any repair.