So many applications are available for OST file Conversion. I would also like to suggest such as OST to PST Converter Software. This app shows all the flexible emails in preview items before saving. In preview items, you can check your select emails. This utility migrates OST to many formats like PST, MSG, MBOX, EML, and EMLX. Download this utility and try a free trial version and migrate a few emails per folder free of cost. The utility supports all MS Outlook versions like 2010, 2013, 2016, 2019, and the latest 2021. Refer:- https://www.softakensoftware.com/ost-to-pst-converter.html