There have been a lot of question reagarding the capabilities of using the MAC Messenger client with OCS 2007.
Messenger 7.0 client is similar to OC client on PC with presence, groups, etc.
You can grab Mac Messenger 7.0.1 here.
Features of the Mac Messenger 7.0 client:
Here is a very useful matrix:
The full article from: The Three UC Amigos