There really wouldn't anything built in to Outlook since OST and PST serve different purposes. If you haven OST, then you most likely were linked to an Exchange account and the OST is just a backup of what's on the Exchange Server. If you use POP/IMAP, the you get a PST file instead..
There are, however, many online sites that claim to do the conversion, trust them with your emails at your discretion and check them out..