There are no free converters that do this. If you have outlook, and access to the mailbox that the OST is attached to, you could use the archiving feature to have Outlook autmatically do the work for you. But if you're trying to convert an OST that you no longer have access to the mailbox for, you're going to have to pay up, as far as I know anyway. This is something that I have researched in the past and was never able to find free or opensource software to do the trick.