Would suggest you to have a look on this article, where they have mentioned some of the best free MS OST Into PST converter tools.
And if looking for manual techniques, you may give a read to this article
Hope these articles help you out
There are various free tools which do the work for you and are quite reliable in converting your OST files into PST formats.
So before going for any paid software, I would suggest you to go for a free software.
One free tool that I have used earlier was Aryson OST to PST converter.
You may just...