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Two letter O separated by a hyphen, recording the short castle.
Also known as: Short castling, Castles kingside, 0-0
O-O records castling on the king's side of the board: the king moves two squares towards the h-file rook and the rook jumps to the far side of it. It is written with letter O in the FIDE Laws, though the digit zero is common in print.
Records castling towards the king's side of the board. The FIDE Laws prescribe the notation and define castling as a single move of the king.
Castling is the only move in which two pieces move at once, and the only one written without naming a destination square — which is why it needs a notation of its own rather than fitting the ordinary pattern.
Two capital letter O shapes joined by a single hyphen, written in place of a destination square.
Count the letters: two for the king's side, three for the queen's. The moves send the king in opposite directions.