Just asking for advice: Is there any reason NOT to install the OCS on the same Machine as Exchange?
I'm running OCS 2007 R2 and Exchange 2010 (including the UM Role)
I'm not sure which versions you are running, but Exchange 2007 or 2010 UM roles will conflict with OCS as both will try to bind to the SIP port. You should be able to mitigate this by binding each to different IPs on the server.
That being said, I am not sure what Microsoft's support policies are regarding such a configuration.