OST files are what was known as offline folders. These contain replicas of the mailbox on the server so as to allow offline viewing. If the OST file were to be corrupted, usually by a bad shutdown. Don’t waste time trying to repair the OST file.
Just recreate another, by unchecking “Cache Mode” and renaming the OST file and enabling “Cache Mode”. A new OST file will be created and the contents of the mailbox replicated. Faster than any OST repair tool.