If you want to Convert OST to PST file format then you can try this third party solution OST to PST Converter Software that will help you to Convert OST file to PST format without any technical expertise.
In this software you can also recover your lost and deleted OST file data with all emails attachments.
It capable to Convert your any size of OST file without any file size restriction.
You can also try this software free trial version and save 30 emails per folder at free of cost.