kingside 0-0 or O-O
0-0 or O-O or 0-0-0 or O-O-O???
Using a capital "O" or a zero "0" for castling notation is acceptable. chess.com uses the "O" for castling; this is probably the "most correct" of the two options, but early notation used to use a zero because computers back then had limited characters for chess notation and computers already had number keys.
Castling notation 0-0 or O-O