If Outlook user wants to convert OST file to PST format, there are few manual trick suggesting by Outlook, which can be used for exporting Outlook mailbox data from OST to PST. Also, many software companies provide a reliable tool for converting OST file data to PST format. I also request you first try manual tricks before using any paid solution. Because it's a free of cost solution.

Learn manual trick steps, here:- http://migrateemails.com/2016/01/23/convert-ost-to-pst-format/