If you want an OST to PST converter, I recommend the ToolsCrunch Mac OST to PST Converter Tool. This is the best solution to transfer emails, attachments, contacts, calendars, and notes from OST to PST files. This app exports multiple and heavy files from OST to PST at once without changing any important data. It is compatible with all versions of Windows OS and MS Outlook. as well as it works on all Mac OS versions, such as 14, 13, 12, 11, 10.15, 10.14, 10.13. It gives the option to recover OST corrupted data. download and free demo version.

You can simply convert PST to MBOX files. I suggest the ToolsCrunch Mac PST to MBOX Converter Tool is the most reliable software to transfer bulk and large emails from PST to MBOX format without changing the original data. This tool migrates emails and attachments from PST to MBOX files without technical interference. It is compatible with all email clients of MBOX, such as Mozilla Thunderbird and Apple Mail, etc. Users can preview the database before transfer and filter selected data. Use the free demo version for testing.