Mountain climbing, plateau, deep diving, space, high altitude (fighter, parachute, some aerobatics), low pressure (the mask popped up by the passenger plane in an emergency is pure oxygen, but it seems that it can only last for 15 minutes), recovery after gas poisoning in hyperbaric oxygen chamber (not counting first aid), and some gas masks are also used for oxygen supply before the college entrance examination.
In short, where there is no oxygen, most places with low air pressure use pure oxygen.