Zillions
Oblique 
. Variant with board with 40 squares turned 45 degrees. By Tony Quintanilla.
- Oblong chess . Variant of Shatranj, played on a four by sixteen sized board. Author: Andrew Juell.
- Oblong Chess 44 . Missing description By Erez Schatz.
- Oblong Shatranj with Die
. Missing description Author: M Winther.
- Octa-Ashtapada. 8x8x8 variant with powerful armies and the potential for multiple royal pieces per side. By Larry L. Smith.
- Octopus Chess . Featuring the Octopus, the Sorcerer Snake, the Ferz Knight, the Wazir Knight and the Ninja Pawns. By Charles Daniel.
- Odds Chess . Ways of giving a weaker opponent better odds. Author: Roger Cooper.
- Odin's Rune Chess . Missing description Author: Michael Nelson. Inventor: Gary K. Gifford.
- OK Chess . After the first move, Pawns can advance any distance. Author: Gary Baydo. Inventor: Clifford Merry.
- Omega chess . Commercial chess variant on board with 104 squares. Author: David Howe.
- Omega Chess Extended . Missing description By Mykola Dolgalov.
- Omega Transplant . The Warmachine Alfil , the Camel Ferz, the Ninja Pawns and the usual suspects battle it out on a new chessboard. By Charles Daniel.
- The One Ring . White wants to get the Ring to the far side and destroy it. By Robert Price.
- One Ring Chess . Chess on a one-dimension ring board. By Larry L. Smith.
- One Way Chess . Missing description By Adrian Alvarez de la Campa.
- Open Chess . Large variant with few pawns. Author: Marek14 . Inventor: Moses Snell-Feikema.
- Operational Chess. Wargame inspired variant played on a wide board. By Andy Thomas.
- Operational Chess 2. Large, wargame inspired variant with ranged pieces. By Andy Thomas.
- Opulent Chess . Zillions file for Opulent Chess. By Greg Strong.
- Orwell Chess . Three player variant themed on George Orwell's 1984. By Glenn Overby II.
- Outback Chess . New pieces on plus-shaped board. Author: Peter Aronson. Inventor: Timothy R. Newton.
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