Oddly enough, in many of the full-motion simulators there's a tank
of O2 that feeds into the cockpit of the trainers. Why, you may ask?
Because when the alarm goes off indicating loss of pressurization,
the pilots grab the fast-donning O2 masks. There were some problems,
such as the fact that the masks fit so well (as they are supposed to
do) that the people couldn't breathe! So when training for loss of
pressure, the staff makes sure the O2 bottles are connected and
turned on.

Acutally, for training purposes, I think they use bottled air,
not O2.