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Three letter O separated by hyphens, recording the long castle.
Also known as: Long castling, Castles queenside, 0-0-0
O-O-O records castling on the queen's side. The rook travels further than in the kingside castle, which is why the notation is longer — a mnemonic that happens to be true rather than a coincidence.
Records castling towards the queen's side of the board, under the notation the FIDE Laws prescribe.
Leaves the king further from the corner than the kingside castle does, and the a-file rook further from the king — a difference of position, not of notation, that the extra character usefully mirrors.
Three capital letter O shapes joined by two hyphens, written in place of a destination square.
Three letters for the queen's side, two for the king's. The longer notation matches the longer rook journey.