A flight attendant turns a lever to activate the oxygen masks, however oxygen masks on jets have a barometric switch that detects sudden changes in pressure and are deployed automatically.
The aircraft is supposedly a Boeing 747 (3-4-3 seating pattern in economy) but the cabin is obviously from a McDonnell Douglas DC-10, with its 2-5-2 seating pattern in economy and its oxygen masks coming out of seat back compartments.
Altimeters on aircraft have the dial ranging from 0 to 9, indicating thousands of feet of altitude, and not 0 to 11. The flight deck's "altimeter" in the movie is actually a novelty clock designed to look like an altimeter.