Make a copy of the ost file first. You can try this setup from this users machine/outlook or on a different machine. Simply copy the ost back into the default location C:\Users\user\AppData\Local\Microsoft\ the outlook folder under app data. Name it the same as the default ost in the folder. Open Outlook, and it should load the old cached version of the mailbox. Then select the emails you want to archive and create a pst. Once complete simply delete the old ost, reopen outlook and import the pst.