It wasn't the oxygen that killed the Apollo 1 astronauts, it was the fire. Oxygen can be toxic at high pressures (which is why deep divers use a mix with helium replacing some of the oxygen) but AIUI breathing 100% oxygen at ordinary atmospheric pressure is fine.
And, relevantly for an article on microdosing, so can anything at high doses, including water (e.g., Jennifer Strange) and oxygen (e.g., Apollo 1).