Lack of oxygen in the air, or carbon monoxide in the air, isn't noticed by people, you simply start falling asleep or act oddly, even hallucinate.
Many "haunted houses" simply had bad flues, carbon monoxide levels would rise with windows closed, carbon monoxide bonds better with red blood cells, so people are getting less oxygen, it would cause them to hear noises or see things that are not there, or simply misinterpret what they are seeing and hearing, and it is real to them, no pills, no pot, no drinks, it is real, but.......it isn't.
Lack of oxygen effects different people in different ways, like drinking alcohol does, not everyone reacts the same way, main difference in lack of oxygen is that you don't know anything is wrong, when you drink you know it will have an effect, you can still make bad decisions though, lol.
It is the slow removal of oxygen or the addition of carbon monoxide, that make people confused, they don't know anything is wrong, they think they are doing the right things while they are doing them.
A person has no clue he is doing the wrong thing.
On the Greek flight they had the pilot on the radio, he had been shutting OFF alarms chasing down a problem that didn't even exist, he talked to ground crews and ignored their questions, he only wanted answers to his questions.
Toward the end of the flight the Co-pilot was seen slumped over in his seat and a fly attendant was seen moving along the aisle and then came into the cabin and sat in the pilots seat, and then waved to the military jet that was pacing the airliner trying to contact it.
Attendant didn't seem to even notice passed out co-pilot, a few minute later air craft ran out of fuel and crashed.
It is a very insidious issue for our brains, the lack of oxygen.
Sudden decompression is a different issue, you are aware of what is happening and can react with generally good judgement.