FAQ: Is it possible to only use 1 server to deploy Office Communications Server?
...deploy Office Communications Server
The simple answer is
If you do not need external users to connect into the OCS service, then it is OK. The minimum setup is
1 Domain Controller
1 Member server installed with OCS
The above will only give you simple MSN-like service within your corporate
However, if you need to have external users to be able...
