If your opponent has castled kingside then opposite side castling can lead to a pawn storm to open up lines with pieces following behind. In this situation, the winner is often the player who has the advantage of time (gets their pawns moving first or maybe already has one or two halfway down the board). Just something to be aware of if considering opposite side castling.
So basically, I love complicating positions and i've noticed that castling queenside makes alot of board positions complicated. What are the downsides and upsides to it and what should i do afterwards?