Have successfully managed to convert an Outlook 2003 OST file to a PST file that can be read in Outlook 2002 or 2003. This is the method that worked for me:
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
This method has converted 2 files both of approx 400Mb.
This process essentially converts an Outlook 2003 file into a 2003 file and “repairs” it to get into a 2002 file. Then you run freeware OST2PST to get it converted an open the 2002 .PST file in Outlook 2003 which is backwards compatible.
I just want a freeware process that will safely convert outlook 2003 .OST files into .PST.