To convert OST to PST without using Outlook, use the free vMail OST to PST Converter for Mac and Windows. You can batch convert an entire Outlook.ost file, including all emails, contacts, calendar events, tasks, and attachments, to a PST file type. You can convert a huge OST data file into PST format without any data loss because there are no file size restrictions. Also, this program is compatible with a number of file types, including PDF, MBOX, EML, MSG, Office 365, Gmail, G Suite, Exchange Server, etc.
Know more info- https://www.vsoftware.org/ost-to-pst-converter.html