O-O and O-O-O, in chess notation

CASTLES

Think of the only moves in chess where the king shifts two squares toward a rook, often used for safety or connecting rooks.

In chess notation, "O-O" and "O-O-O" represent kingside and queenside castling, where the king moves two squares toward a rook. The answer "CASTLES" plays on the word "castling" while also referencing the rooks (which look like castle towers). It’s a clever double meaning—both the chess move and the literal structures.