It’s really several factors. Supplemental oxygen is common for people with diminished lung capacity, carbon monoxide exposure etc. However long term it’s not a good idea for healthy people. https://en.wikipedia.org/wiki/Oxygen_therapy

At low pressure pure oxygen can similarly be beneficial, mountain climbers eventually need supplemental oxygen for Mount Everest though a few have made the trip without it they can’t stay at that altitude indefinitely. It can even help on airplane flights as commercial airlines don’t set things to sea level.

Where healthy people run into issues is when partial pressures get well over 100% at sea level. Part of the issue is people adjust their breathing based on carbon dioxide not oxygen levels. So at say 10 atmospheres at normal atmospheric mixtures your breathing the equivalent of 210%, but you don’t slow down enough to compensate. Thus why divers care so much about gas mixtures, however people with diminished lung capacity are going to encounter issues at different levels than normal divers.