AO
..is short for ‘Adults Only’ (most extreme videogame rating in the U.S.). See also the other vg ratings: EC(Early Childhood), K-A(Kids to Adults; was replaced by 'E' in 1996), E(Everyone), E10+(Everyone ten years of age or older), T(Teen), M(Mature Audience), AO(Adults Only), and ESRB.
Example:
GTA:SA(Grand Theft Auto: San Andreas) is no longer rated 'M', but instead 'AO', and subsequesntly is no longer on the shelves, barely in stock at all.
GTA:SA(Grand Theft Auto: San Andreas) is no longer rated 'M', but instead 'AO', and subsequesntly is no longer on the shelves, barely in stock at all.