Previously, installing OCS 2007 R2 roles on a server with Windows Server 2008 R2 operating system was not supported. Microsoft has now announced that OCS 2007 R2 finally supports 2008 R2 OS. It’s worth to note that the Group Chat Server role still does not support 2008 R2. Upgrading an existing installation to Server 2008 R2 is not supported either, so it looks like this support only applies to new OCS 2007 R2 installations.
More info here: http://support.microsoft.com/default.aspx/kb/982021?p=1
It’s also also worth to note that Server 2008 R2 domain also is supported. If you introduce 2008 R2 domain controllers to an existing domain, you’ll have to run forest prep again.
More info here: http://support.microsoft.com/kb/982020