Oxygen under pressure...
With regard to being 'force-fed' oxygen and high cabin altitudes, it is very dangerous to wait until 40,000 feet to breathe oxygen under pressure. The USAF figures that you'll begin to lose ground starting at about FL250, with things gradually becoming more critical as altitude increases. At a point about FL350, without being force fed, you'll be in trouble. Fortunately, the emergency descent will bring you down to reasonable levels quickly, and it's not a problem.
With reference to FCOM 1.35.20 Page 7, the 320 series Airbus I fly has a mask that automatically takes care of this problem. "Note: Overpressure supply is automatically started, when the cabin altitude exceeds 30,000 feet."
Tom