0-0 or O-O or 0-0-0 or O-O-O???

Castling notation 0-0 or O-O

kingside 0-0 or O-O

Queenside O-O-O or 0-0-0

O-O/O-O-O

Ok

But is it OK? I think the guy's answer is wrong.

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.

Interesting quote from chess historian Graham Burgess about using capital O:

"Presumably these were initially just one individual's personal foible (or error) in the early days of chess on the Internet, but the standard is now established. "

OTB is zero, it's what's described in the standard. But PNG uses O, which is what online solutions copy. So I'd say it depends on the context