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O

Obstructive Sacrifice: A material sacrifice to hinder an opponent's development.

Occupation: A Rook or Queen that controls a file or rank is said to occupy that file or rank.   A piece is said to occupy the square it is sitting on.

Official Rules of Chess: Official FIDE publication setting forth the Laws of Chess .

Open: Short for Open game or Open file. Also refers to a type of tournament in which any strength of player can participate.

Open file: A file cleared of Pawns. It offers a corridor for attack, especially if occupied by doubled Rooks. A file is still open even if it is occupied by pieces other than Pawns.

Open game: A position characterized by many open ranks, files, or diagonals, and few center Pawns.

Open Tournament: A tournament which is open to any player.

Opening: The start of a game, incorporating the first dozen or so moves. The basic goals of an opening are to develop pieces as quickly as possible.

Openings: The more-or-less standardized and analyzed patterns of moves that both sides make at the start of a game. Some are named after people (Ruy Lopez), some after places (Budapest Counter-Gambit), some after pieces or moves (Four Knights Defense). Some are descriptive (Giuoco Piano, or quiet game).

OTB: Abbreviation for Over-The-Board.

Opposition: A position in which opposing Kings stand on the same rank, file or diagonal, separated from each other by only one square. The player whose move brings the Kings into opposition holds an advantage that, in an end-game, can be decisive.

Outpost: Term coined by Nimzowitsch; a piece placed on a square (on an open or half-open file) on the opponent's side of the board, protected by a Pawn, which cannot be attacked by an enemy Pawn. The power of the piece on the outpost can be so strong the opponent may be forced to exchange it, even at the cost of material or positional loss.

Outside Passed Pawn: A passed Pawn away from most of the other Pawns on the board.

Over the Board: A description of games played face to face, as opposed to correspondence Chess.

Overload: A situation where a Pawn or piece must perform too many defensive functions, so that if one it is forced to perform one function a weakness will be created.

Over-protection: Nimzowitsch's concept of concentrating many pieces

and/or Pawns--even more than might seem necessary--on an important square. This creates a strong square which interacts beneficially with the over-protecting pieces.

Overextension: When space is gained too fast. By rushing his Pawns forward and trying to control a lot of territory, a player can leave weaknesses in his camp or can weaken the advanced Pawns themselves. He is then said to have overextended his position.

Overworked piece: A piece that is required to single handedly defend too many other pieces.