Similar problem with my Dad. Started smoking in WWII, 2 packs a day for the next 50 years. He was told he needed oxygen 24/7, but when he would use it, he felt fine and would decide he didn't need it, take it off, then get "foggy" (for want of a better term) and not realize he needed the oxygen, thus damaging his brain cells even more. Heart problems also reduced blood oxygen, along with COPD from the smoking.

I do not smoke.

Moral: if your elders need the oxygen, make sure they use it 24/7 (or as directed).