There are some apps that can convert .olm files to .mbox files for migration from Outlook to Apple Mail. Some are available in the Mac App Store. I have not used any of them, so I cannot recommend one.
The "manual" method to move your mail from Outlook to Apple Mail would be to drag your folders (one at at time) from the Outlook folder pane to your desktop (or other location on your drive). Do this one folder at a time. It will create an MBOX file for each folder. But be aware that MBOX files do not work with nested folders, you have to do each folder individually (if you have nested email folders in Outlook it would be best to "un-nest" them first).
Once you have your MBOX files you can import them into Apple Mail. (Mail > File > Import Mailboxes > Files in MBOX format).
I suggest trying one folder first. If that migrates successfully, then do the rest of your folders.