One of the earliest findings of the investigations into oxygen that O2 is toxic in excess.

Space-cabin Atmospheres: Oxygen toxicity (1964) (google.com) - https://news.ycombinator.com/item?id=25883728

The Haldane Effect and the Bohr Effect are the central findings that explain the transportation and exchange of carbon dioxide and oxygen: https://en.wikipedia.org/wiki/Haldane_effect / https://en.wikipedia.org/wiki/Bohr_effect

People who hyperventilate exhale too much carbon dioxide, which disrupts the acid/alkaline balance of the blood and makes it harder for the body to use the oxygen available to it.

The Apollo capsules were originally spec'd to use a mixed gas atmosphere. When the capsule got too heavy they switched to using a pure-O2 atmosphere. After the Apollo 1 fire they switched to using an atmospheric mix (80/20 N2/O2) at launch, which gradually changed to pure oxygen as the flights progressed.

Treatment with pure oxygen is not helpful for sick people:

Mortality/morbidity: acutely ill adults liberal vs. conservative Oxygen Tx (2018) (thelancet.com) - https://news.ycombinator.com/item?id=22993262

I typed up some notes about oxygen toxicity: https://www.taxiwars.org/2021/06/folly-medical-hyperventilat...

Medicine decided that the antidote to oxygen toxicity didn't need to be used anymore in the mid-1950's.