Office Communications Server (OCS) is a standard based SIP server that supports several real-time communications features such as presence-based VoIP call management, instant messaging capabilities, and video, audio and web conferencing. A possible point of differentiation between CallManager and OCS is that CallManager maintains the state of the calls in progress to support mid-call features such as barge, shared line and boss/administrative functions.

http://cisco.com/en/US/docs/voice_ip_comm/cucme/pbx/interop/notes/617030nt.pdf