- Oathcircle [[MUX] ] (February 19, 2013)
- Olympus Island MUSH [[MUSH] Penn 1.7.7p28] (June 24, 2004)
- Once Upon A Time... Steampunk! [[LP]] (February 28, 2011)
- OtherSpace [[MUSH] PennMUSH] (May 7, 2012)
- Otherworld, the land of Tirn Aill [[Circlemud] Customized] (April 16, 2012)
- OurPlace MUD [[Envy] Heavy Modified] (April 23, 2009)
- OuterSpace [LP MUD] (October 1, 1999)
- Overdrive [[LP] ] (May 17, 2013)
- OzMUSH [[MUSH] ] (August 18, 2011)
- The Oaktree-house talker [Nilex] (May 25, 1998)
- The Outlands - 4th Age [[Rom] 2.4 ( Heavily Modified Tartarus )] (August 7, 2007)
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