A MOO (MUD, object-oriented) is a text-based online virtual reality system to which multiple users (players) are connected at the same time.

Moo or MOO may refer to:

• MOO, a type of text-based online virtual reality system
• MOO (programming language)
• Moo.fx, a JavaScript effects library
• Mode of operation in cryptography

Master of Orion (MoO or MOO) is a turn-based, 4X science fiction computer strategy game released in 1993 by MicroProse on the MS-DOS and Mac OS operating systems. The game is the first in its franchise, and the rights are now held by Wargaming.net. The player leads one of ten races to dominate the galaxy through a combination of diplomacy and conquest while developing technology, exploring and colonizing star systems.