OSCAR is
AOL's
instant messaging protocol standing for
Open
System for
Communication in
Realtime. Despite its name, the specifications for the protocol are far from open. In fact, AOL has gone to great lengths to keep
competitors from implementing compatible clients for their proprietary messaging system.
Currently OSCAR is in use for AOL's two main systems: ICQ and AIM. Large parts of the protocol are nowadays understood after reverse-engineering the protocol, implemented by an ever-increasing number of clients.