Reading this article seems to explain the hypoxia.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1114067/
"However, it is difficult and expensive to arrange oxygen supplies so flow rates are kept low. The oxygen is used when sleeping, normally at 1-2 l/min via a face mask, and when climbing...
There seem to be a significant number of reports of hypoxia from people climbing Everest on 100% oxygen, but I suppose this could be because of a limited flow rate I suppose compared to the full flow rate used in diving, or perhaps other physiological issues resulting from the low pressure...
Hi, I'm currently learning to scuba dive and we use a very simple way to calculate the partial pressure of oxygen at depth to avoid oxygen toxicity, for example at sea level of 1 atm and 21% O2 concentration the partial pressure is calculated to be 0.21, and at 60m (pressure increases by 1 atm...