If you are upgrading to Outlook 2021 on Windows, converting OLM to PST is necessary. PDS OLM Converter creates PST files that are fully compatible with the latest Outlook editions.
You can use MS Outlook's "Import/Export Wizard" to export OST emails directly to PST, or you can use a tool like FixVare OST to PST Converter to do the same thing. This method keeps the folder structure and needs access to the original Outlook profile. The program can quickly and efficiently convert a number of OST files to PST format without losing any data. The tool works with all versions of Windows and comes with a free trial.
The OST to PST Converteris a simple utility for converting Outlook emails to PST format while preserving the original file structure. The programme's aim is to transfer files. A variety of document types are also supported, including HTML, EML, MBOX, and more. The utility's user-friendly interface makes it accessible to anyone, and it is designed to work with the least amount of hardware and software configuration, giving users the best interaction possible on any system.
If you are having bunch of OST files to be exported in PST format on macOS then I suggest you to try direct solution i.e. MacMister Mac OST Converter Software. The software migrate bulk of OST files automatically to PST file without any alteration in mail properties. Learn more detail with the below link: https://www.macmister.com/blog/convert-ost-to-pst-without-outlook/