Starts with the letter 'O'
- Objective Bastardo. Cut-throat four-player variant where each player has a secret objective. Author: Ed Friedlander.
- Once More, with Deans. Subvariants of River-with-university-and-cathedral series with extra piece type. By Charles Gilman.
- Orwell Chess. Three player variant themed on George Orwell's 1984. (7x12, Cells: 84) By Glenn Overby II.
- Orwell Chess . Three player variant themed on George Orwell's 1984. By Glenn Overby II.
Total items listed: 4