Also sometime this method that I found at another site:
I Have successfully managed to convert an Outlook 2007 OST file to a PST file that can be read in Outlook 2007 or 2010. 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 2007 (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