The .ost is only a copy of your exchange mailbox stored locally on your pc when you are running in cached mode. If it becomes corrupted you can safely delete the .ost file from your computer. Take your account off of cached mode, then close outlook. Delete the .ost file. Re-start Outlook and verify all is well. Then put Outlook back on cached mode if that is how you prefer to run Outlook. The .ost file will automatically be re-built anew. You do not HAVE to run in cached mode at all. I finally took my Outlook off of cached mode after the .ost got corrupted for the umpteenth time. Deleting your .ost does not affect your live account, again it is only a copy created for running in cached mode. Oh and by the way, there is no longer a 2 GB pst limit in Outlook 2007, that was changed in 2003 when the pst was changed to unicode format.
First of all make the Copy of OST file
and then proceed to deletion, sometimes technology misbehaves and might lead to unwanted problems like inaccessibility of email backup file. You can try this program OST Recovery software
it can recover all database of OST file, calendar, notes, contacts, attached files and customized folders.