Microsoft released OneNote for OS X today - you can find it in the Appstore using this Link:
Searching the AppStore did not work for me - but that obviously should be fixed soon.
So download OneNote and enjoy! I prefer it over Evernote and the missing Mac version was the only reason for me, why I did not use it yet for all of my note taking - those days are finally over.
Great Product - great Price. Thanks Microsoft.