The game isn't going to have an AO rating. Many high-profile retailers simply refuse to sell those. Not gonna happen.
Besides, 18+ provides all the mature content needed. It allows for very copious amounts of gore, all the difficult/incomfortable themes you could ever want, as much swearing as one can stomach, and plenty of sexual content, just not up to fully animated penetration. The only way it gets AO is if it features really exaggerated violence, such as SAW-style torture porn or, well, actual pornographic imagery. Neither of which fit Mass Effect as I see it.