(MOO) One of the many MUD spin-offs (e.g. MUSH, MUSE, and MUX) created to diversify the realm of interactive text-based gaming. A MOO is similar to a MUSH in that the users themselves can create objects, rooms, and code to add to the environment.

The most frequently used server software for running a MOO is LambdaMOO but alternatives include WinMOO and MacGoesMOO.