Moving from Outlook 2011 back to Mail. How?

I thought I would like to use Outlook 2011 for the mac. It is great software, but I would prefer to move back to the native OS X Mail app. It just makes syncing so much easier with my devices.

However Outlook only produces OLM files... How do I get messages back over to Mail. I have looked everywhere and cannot find a solution.

Any info would be greatly appreciated. Thanks.