ScanOST Vs. ScanPST

Solution 1:

Scan OST is only useful if the user in question is not able to come into the office. Simply deleting the OST file (while outlook is shutdown) and restarting outlook will cause it to recreate the file. If a user is working offline and has an issue scan OST will recover the offline mail. I would say that the first time there is an issue with a users OST file you shouldn't have to scan the PST as well. However whatever corrupted the OST has a good shot at screwing up the PST file as well (although the PST file format is a litte more resilient). So when they call you a second time scan both.

Solution 2:

The Inbox Repair tool ScanPST.exe works on Microsoft Outlook 97-2002 Personal Folders File (.pst) and the Office Outlook Personal Folders File (.pst) data files in Microsoft Office Outlook 2003 and Microsoft Office Outlook 2007.

The OST Integrity Check tool ScanOST.exe runs only on .ost files, which are Outlook Offline Folder files (synchronized offline Exchange repositories) and can be used to diagnose and repair synchronization issues. It scans both your .ost file and your mailbox on Exchange, compares objects and folders in each, and attempts to reconcile synchronization differences between them.

While ScanPST can run offline, you must be connected to your Exchange server in order to run ScanOST.

If you cannot open your .ost file, you can run ScanPST to diagnose and repair the corrupted .ost file. ScanPST can be used on your .ost and .pst files.

This Microsoft Office Support article contains further details: Scan and repair corrupted Outlook data files