Restoring ost-files
Restore Ost-files aren’t intended to be backed up and restored. However, under certain circumstances an ost-file can still be used directly to restore data.
You can restore an ost-file when;
When the IMAP account the ost-file belongs to is still configured in Outlook and you…
haven’t removed and re-added the IMAP account.
haven’t created a new Mail Profile
aren’t trying to use it for the IMAP account on another computer.
When the Exchange account the ost-file belongs to is still configured in Outlook. This can also be in a new Mail Profile or on another computer as long as you have connected to the Exchange server at least once.
To restore data from the ost-file;
Close Outlook.
Rename the current ost-file of the account to .old.
Restore the ost-file to the location of the current ost-file and rename it if needed.
Disconnect yourself from the network to make sure that no changes are being synched when the account reconnects. This could for instance empty the ost-file if the data was no longer on the server.
Start Outlook.
Export any data that you wish to keep to a pst-file.
Close Outlook.
Delete or rename your recovered ost-file. If your original cache was still working, you can rename it back from .old or otherwise make sure there is no longer an ost-file for your account.
Reconnect yourself to the network.
Start Outlook.
Once Outlook is done synching, you can import the data from the pst-file.
Source: https://www.howto-outlook.com/howto/backupandrestore.htm#restore-ost