Fastest O-O-O

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I know at least one common opening variation where White castles O-O in four moves, the minimum number of moves in which it can be done. In theory, castling O-O-O can be done in five moves, but I can't think of a common variation where it actually occurs. (I can invent a move sequence where O-O-O is played on the fifth move, but I'm looking for a 'real' variation, something that a good player might use in an important game.) So here's the question: In what variation(s) is castling O-O-O played the fastest for White? For Black? - Mark