O2 is a new communication protocol and implementation for music systems that aims to replace Open Sound Control (OSC). Many computer musicians routinely deal with problems of interconnection in local area networks, unreliable message delivery, and clock synchronization. O2 solves these problems, offering named services, automatic network address discovery, clock synchronization, and a reliable message delivery option, as well as interoperability with existing OSC libraries and applications. Aside from these new features, O2 owes much of its design to OSC and is mostly compatible with and similar to OSC. O2 addresses the problems of inter-process communication with a minimum of complexity.