OST files are linked to the MAPI mail profile on the PC. Delete the MAPI profile, and the OST is toast. Likewise, you cannot attach the OST of one MAPI profile to a different profile.
An OST is just a synchronised copy of the mailbox (we're talking Exchange here). So unless the user has processed and written some mail offline, there shouldn't be anything worth saving in the OST. In cases when you do need to save offline worked-on or generated emails, the procedure is: start up the profile in offline mode; add a PST (personal folders) to the profile; drag/copy/move the email you want to save to the PST; detach the PST from the profile; log out. Now the PST has all the mail you've saved, and can be added into a new, different profile (PSTs aren't linked to MAPI profiles or even mailboxes) and the mail recovered that way.