OSCAR is AOL's instant messaging protocol standing for Open System for Communication in Realtime. Despite its name, the specifications for the protocol are proprietary. AOL has to gone to great lengths to keep competitors, namely Microsoft and Cerulean Studios, from implementing compatible clients for their proprietary messaging system.
Currently OSCAR is in use for AOL's two main instant messenging 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.