You can easily batch convert OST files to PST format by using mostly used ZOOK OST to PST Converter tool. It allows users to convert OST files to PST format including emails, contacts, calendars, notes, journals, etc. This tool is designed or developed in such a way that anybody can easily convert countless OST files to PST format without any extra efforts. It is fully supportable to all available versions of Microsoft Outlook and Windows Operating Systems.

https://www.zooksoftware.com/ost-to-pst/