(if you don’t know what an OST file is, see below)

There are four options, one of these below might work for you.
NOTE: This is a work in progress post.  It will be updated when I learn something.

The OST2PST conversation method.

1. Backup ost file(s)
2. Rename outlook.ost to outlook.pst
3. Use scanpst.exe from Outlook 2002 (in C:\Program Files\Common Files\System\Mapi\1033) to repair outlook.pst
   (You can always copy scanpst.exe from an alternative installation, include MAPI32.DLL and GAPI32.DLL)
4. Rename outlook.pst back to outlook.ost
5. Run OST2PST which will then convert the file successfully to outlook.pst
6. Outlook can then open the outlook.pst file

Sources:
MS Exchange Blog: OST to PST
Geeks To Go: Converting Outlook .OST files to .pst

This method may not work with Outlook 2003 or later OST files.

Modify the header file method
The type of Outlook file, whether it is a PST or OST, is stored in the header of the file.
It might be possible to modify the header, and open the modified file in Outlook.

The file format has been documented in “Personal Folder File (PFF) forensics” by Joachim Metz.

Compile and use the pffexport program method
“Extracts items from a Personal Folder File (OST or PST)”.  See the libpff site.

Buy any number of commercial products method.
I’ve tried none of these.  You are on your own.

So what is an OST file then?
If you’re asking this question, you probably don’t need to know.  🙂

An Outlook Offline Storage Table (OST) file is a copy of your Exchange email account.  This copy allows you to access your (offline) email while you are not connected to a network.  Such as when you are travelling..

Update October 2009: Roadmap for Outlook Personal Folders (.pst) Documentation