You can easily convert Mac OST files using MacUncle OST to MBOX Converter. The tool supports Mac OS Machines.
Start MacUncle OST to MBOX Converter on your Mac Computer.
Select OST file using Add File(s) or Add Folder(s).
Choose saving format as MBOX.
You can also select the File Naming Option and Choose Destination path if required.
Mac OST conversion process completed successfully, you can see the message of completion.