You can try the software
OST to PST Converter tool. I hope this utility could help you out to transfer OST files to PST format. The software is effective and takes very less time in the whole conversion. This application has one of the most simple and user-friendly interfaces. Free Trial version which transfer the first 10 items from each folder.
More info visit here:-
https://www.toolsbaer.com/ost-to-pst-conversion/