The following fixes may help resolve the Outlook problem "OST File is in Use and Cannot be Accessed":

Turn off the cashed exchange mode.

When Outlook is open, select the File tab.
To select your email account, select Account Settings.
Uncheck the Cached Exchange Mode option by clicking on Change.
Select Next, Continue, and Exit. To make the changes take effect, restart Outlook.
Terminate Outlook processes in Microsoft:

To access the Task Manager, use Ctrl+Alt+Delete.
Check for processes with the names msimn.exe or outlook.exe.
Click End Process after selecting each one.
Go back to opening your OST file after closing Task Manager.

More Ways to Troubleshoot OST File is in Use and Cannot be Accessed

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.