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.
See Also
 
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