Starts with the letter 'O'
- OK Chess . After the first move, Pawns can advance any distance. Author: Gary Baydo. Inventor: Clifford Merry.
- One Way Chess . Missing description By Adrian Alvarez de la Campa.
- OOmost Chess. All movement in this variant is a form of castling. (8x8, Cells: 64) By Ralph Betza.
- Orda Chess. Asymmetric variant where one army has pieces that move like knights but capture differently. By Daniel Lee.
- Overkill Chess. Paired pieces are compounds of 2 FIDE elements, King's partner of all three. (8x8, Cells: 64) By Charles Gilman.
- Overprotection Chess. If an attacked piece is more often defended than it is attacked, it gains extra powers. (8x8, Cells: 64) By Ralph Betza.
|4 'Game' entries listed||2 'Zillions of Games file' entries listed|
|1 'Each player has a different army' entry listed|
Total items listed: 6