OSCAR is AOL's flagship instant messaging and presence information protocol standing for Open System for CommunicAtion in Realtime. Currently OSCAR is in use for AOL's two main instant messaging systems: ICQ and AIM.
Despite its name, the specifications for the protocol are proprietary. AOL has gone to great lengths to keep competitors, namely Microsoft, Jabber, and Cerulean Studios, from implementing compatible clients for their proprietary messaging system. In 2002, AOL signed a contract with Apple Computer, allowing them to use it (as well as AOL's own code libraries) in their iChat program. This allows iChat users to interact with ICQ and AIM users directly. As a part of this deal, Apple's .Mac service could hook into the service by allowing .Mac members to log in using their .Mac accounts (in the form of the full e-mail address ? username@mac.com) on the AIM network.
Large parts of the protocol are nowadays understood after reverse-engineering the protocol, implemented by an ever-increasing number of 3rd party clients.
