Open System for Communication in Realtime (OSCAR) is AOL's proprietary instant messaging and presence information protocol.
AOL has gone to great lengths to keep competitors (namely Microsoft, XMPP, and Cerulean Studios) from implementing compatible clients for their proprietary messaging systems.
The deal also permitted users of Apple's .Mac service to log in to the AIM network using their .Mac email addresses.
As of 2009[update], large parts of the protocol had been reverse-engineered and implemented by a number of third-party clients.
In fact, AOL expands on the general concept of OSCAR, adding systems for service-side buddy lists and icons, as well as features for file transfers using Bonjour.