Glossary of Chess

Odds (handicap): This refers to the stronger player giving the weaker player some sort of advantage, such as in material, extra moves, time on the clock, or some combination thereof. Since the advent of the chess clock, time odds have become more common than material odds. The stronger player will begin the game with only one or two minutes on the clock, while the weaker player is given five or more minutes on the clock.
Offhand game: See skittles.
Olympiad: An international team chess tournament organized biennially by FIDE. Each team represents a FIDE member country.
O-O: This term is a common abbreviation for kingside castling.
O-O-O: This term is a common abbreviation for queenside castling.
Open file: A file on which there are no pawns. A file on which only one player has no pawns is said to be half-open.
Open game: 1. A game in which exchanges have opened files and diagonals, as opposed to a closed game.; 2. A chess opening that begins with the moves 1.e4 e5 (which is also called a Double King Pawn opening). See also closed game and semi-open game.
Open tournament: A tournament where anyone can enter, regardless of rating or invitation. (Compare to closed tournament.)
Opening: The beginning moves of the game, roughly the first 10-20 moves. In the opening players set up their pawn structures, develop their pieces, and typically castle. The opening precedes the middlegame.
Opening preparation: Home study and analysis of openings and defenses that one expects to play, or meet, in later tournament or match games. In high-level play, an important part of this is the search for theoretical novelties that improve upon previous play or previously published analysis.
Opening repertoire: The set of openings played by a particular player. The breadth of different players' repertoires varies from very narrow to very broad. For example, a player who always opens with 1.e4; always meets 1.e4 with the Sicilian Defence, and the Najdorf Variation of it if allowed; and always meets 1.d4, 1.c4, or 1.Nf3 with 1...f5, intending to play the Dutch Defence, has a very narrow opening repertoire. Bent Larsen, who opened at various times with 1.e4, 1.d4, 1.c4, 1.f4, 1.Nf3, 1.b3, and 1.g3, and played a large number of different defenses as Black, had a very broad opening repertoire.
Opening system: An opening, such as the Colle System or Hippopotamus Defence, that is defined by one player's moves, which can be played regardless of the moves of the opponent.
Optimal play (best response): Optimal play is when both sides make their best move at each turn, or one of equally good alternatives. One side tries to win as quickly as possible while the other side tries to delay it as long as possible, or optimal play may result in a draw.
Opposite color bishops: See Bishops on opposite colors.
Opposition: A situation in which two kings stand on the same rank, file or diagonal with one empty square between them. The player to move may be forced to move the king to a less advantageous square. Opposition is a particularly important concept in endgames.
Outside passed pawn: A passed pawn that is near the edge of the board and far away from other pawns. In the endgame, such a pawn often constitutes a strong advantage for its owner.
Outpost: An outpost is a square protected by a pawn that is in or near the enemy's stronghold. Outposts are a favourable position from which to launch an attack, particularly using a knight.
Overextended: A position where a player has moved a piece or group of pieces (usually pawns) away from the rest in such a way that they are too difficult to defend.
Overloading: A piece that has too many defensive duties. An overloaded piece can sometimes be deflected, or required to abandon one of its defensive duties.
Overprotection: The strategy of protecting a pawn or specific square of the chessboard more than is immediately necessary. This serves to dissuade the opponent from attacking that specific point and provides greater freedom of movement for the pieces protecting that square. This can cause an opponent to pursue a faulty plan or no plan at all. Aron Nimzowitsch was one of the foremost proponents of overprotection.
Over-the-board (OTB): A game is said to be played over-the-board if opponents play the game face-to-face as opposed to online chess or correspondence chess.
Overworked: Another term for Overloaded.