The point of having an AO rating is so that we don't get ridiculous cases like Fallout 3 being initially banned in Australia because of the use of syringes and pills (the gore was ok, however!). The drug references were deemed too strong for a 15+ rating. With an 18+ rating, however, it's likely that Fallout 3 would have been accepted for release without needing any adjustments to its content.
So it's not a "AO" rating, it's an "M" / PEGI 18 rating.
("AO" in US terms is the game equivalent of X or NC-17 rated movies, not R)