Starts with the letter 'O'
- ObstructionChess. A new chess variant designed to challenge old strategies and improve your chess play with logical and organic obstructions. (8x8, Cells: 64) By martin rinab.
- Once More, with Deans. Subvariants of River-with-university-and-cathedral series with extra piece type. By Charles Gilman.
- One King Shogi. Checkmate the neutral king. (9x9, Cells: 81) By Daniil Frolov.
- Onitama. Cards decide how pieces move, perfect information with no chance beyond setup. Author: Mikk . Inventor: Shimpei Sato.
- Open King 4*4DChess. Four dimensional game with 48 pieces. (4x4x16, Cells: 256) By Kevin Pacey.
- Open King 5*4DChess. Four dimensional game with 74 pieces. (5x5x25, Cells: 625) By Kevin Pacey.
- Option Chess. Missing description (8x8, Cells: 64) Author: (zzo38) A. Black. Inventor: Paul Bonham.
- Óskar's 3dchess. Missing description (8x8x5, Cells: 320) By Hafsteinn Kjartansson.
- Óskar's 3dchess version 2. Missing description (8x8x5, Cells: 320) By Hafsteinn Kjartansson.
- Overkill Chess. Paired pieces are compounds of 2 FIDE elements, King's partner of all three. (8x8, Cells: 64) By Charles Gilman.
- Overkill Ecumenical Chess. Compounds of 2, 3, and all 4 Wildebeest Chess elements. (9x10, Cells: 90) By Charles Gilman.
- Overyang. Variant using compounds of Yang Qi pieces. (11x13, Cells: 143) By Charles Gilman.
Total items listed: 12