Inventors Starting with O
- Tim O'Lena [info]
Tim's 3d Chess. On a 5x5x5 board with two kings per player. (5x(5x5), Cells: 125) 
Tim's 3D Chess. On a 5x5x5 board with two kings per player.
Tim's 3D Chess. On a 5x5x5 board with two kings per player., more...
- Patrick O'Neal [info]
Slide-shuffle. Variation of Shuffle Chess with special castling. (8x8, Cells: 64)
- Steffan O'Sullivan [info]
Steffan O' Sullivan's Three-players Chess. Three player chess variant on a board with 58 squares.
- Peter Obermair [info]
Delayed Chess
. Only see your opponent's moves that are two moves old.
- Jesse A. Obligacion [info]
Chess II. Two extra files with two additional pieces (princes) on each side. (10x8, Cells: 80) Chess II 
. Two extra files with two additional pieces (princes) on each side.
- King Gustav III of Sweden [info]
Gustav III Chess. Play this historical variant with Amazons in extra corner spaces. Gustav III's Chess. Invented by King Gustav III of Sweden (1746-92).
- Mikko Oksalahti [info]
HiddenLynx. A beautiful, user-friendly, Windows program for Chinese Chess.
- Alexandr Oleshko [info]
Gess on zillions game. Gess zillions rules. Chess Remix. Create your chess variant. ()
- Tibor Orban [info]
Chess problems: A proof game. Problem to construct a legal game of given exact length to reach a given position.
- Torben Osted [info]
Transporter Chess. In addition to the usual set of pieces, players have a transporter piece, which can transport pieces one square in any direction. (8x8, Cells: 64)
- Glenn Overby II [info]
Abecedarian Big Chess (ABChess). Buy-your-own-army variant on a big board; 26 piece types. (11x11, Cells: 121) Beastmaster Chess. Large chess variant with a fantasy theme, emphasizing leaping pieces. (8x11, Cells: 84) Glenn's Decimal Chess. A 10x10 blend of FIDE, Shogi, and Xiangqi influences. (10x10, Cells: 100) , more...
- William Overington [info]
Herb garden chess. Variant on 7 by 12 board with additional combination pieces. (12x7, Cells: 84) Tree garden chess. Large chess variant on 10 by 10 board with 4 by 4 area missing from the middle. (10x10, Cells: 84) Tree Garden Chess. Zebras, camels, cardinals, marshalls negotiate around a central lacuna., more... - Patrick O'Neal [info]