To convert your OST data into PST format using free methods, I suggest that you read the suggested blog that contains free manual methods. In this blog, four manual methods are described that you can try depending on your needs. The author described the methods in a simple way and told about all the features and limitations of these methods.

To try the methods, you can visit: www.datarepairtools.com/blog/free-methods-to-convert-ost-to-pst-file/