To fix the "OST File is in Use and Cannot be Accessed" error in Outlook, you can try the following solutions:

Disable Cached Exchange Mode:

Open Outlook and click on the File tab.

Select Account Settings and choose your email account.

Click on Change and uncheck the option for Cached Exchange Mode.

Click Next, Finish, and Close. Restart Outlook to apply the changes.

End Microsoft Outlook Processes:

Press Ctrl+Alt+Delete to open the Task Manager.

Look for processes named outlook.exe or msimn.exe.

Select each one and click End Process.

Close Task Manager and try opening your OST file again.

Recreate the MS Outlook OST File:

Close all applications using Outlook.

Restart your computer.

Locate and delete the existing .ost file on your computer.

Open Outlook again to let it create a new OST file.

Disable Outlook Add-ins:

Open Outlook and go to the File menu.

Select Options, then click on Add-Ins.

Choose COM Add-Ins from the drop-down menu.

Uncheck any add-ins causing errors and click OK.

Close and reopen Outlook to check if the error persists.

Repair Outlook OST File Using the Inbox Repair Tool (SCANPST.EXE):

Close Outlook and locate SCANPST.EXE on your computer.

Run SCANPST.EXE and select your OST file for scanning.

If errors are found, choose to repair them.

Restart Outlook after the repair process completes.

These methods should help resolve the "OST File is in Use and Cannot be Accessed" error in Outlook. If the issue persists, consider seeking professional assistance or using specialized OST recovery tools.