If you are looking for the game "Master of Orion", abbreviated "Moo" or "MOO", see Master of Orion. For other uses, see Moo.

MOO is short for MUD object oriented and is a type of MUD text-based virtual reality system, created by Stephen White (aka Ghondahrl aka ghond) in 1990. Examples of MOOs include LambdaMOO, NowMOO , MediaMOO, and LinguaMOO. MOOs can be programmed using the MOO programming language. Some MOOs are used in academic environments for distance education or collaboration but others are primarily social in nature, or used for role-playing games (RPGs). Many MOOs are now web-based, integrating the text environment with multimedia features. MOOs have been developed to such an extent that one MOO can even be accessed by Microsoft's MSN Messenger. An example of a MOO that has this feature is ScriptersMOO.

There are a number of moo programming resources. One definitive guide is the LambdaMOO Programmer's Manual by Pavel Curtis, 1997.