During WWII as aircraft became capable of higher altitudes some pilots reported becoming "sluggish" after a time aloft (or maybe it was their buddies reporting on them). With current knowledge, this would be an expected consequence of prolonged low oxygen. Quite a number also reported pain like "the bends" on rapid ascents to altitude.

While it was quite a while before on-board oxygen was supplied for sustained flight at high altitudes, it was common practice during a brief intermediate period for air crews to breath pure oxygen for an hour before takeoff. The pure oxygen allowed exhaling some of the nitrogen normally absorbed in the blood without it being replenished. It's the nitrogen bubbles that are primarily the cause of "bends." The blood carries only the oxygen that's absorbed into the hemoglobin, so it's not released as readily as the nitrogen, which is simply "dissolved gas," when ambient pressure is reduced.

US astronauts have commonly used the same "pre-exposure" to pure oxygen prior to EVA activities, to reduce the susceptibility of bends. I don't know that it's still the practice, but suspect that it is. Some early US and Russian space trips used pure oxygen continuously for the duration of the "flights."

Pure oxygen is not "poisonous" in any real sense of the usual uses of the word. In some people, prolonged breathing of pure oxygen can cause imbalances in other absorbed gases and in electrolytes in the blood and tissues, and those imbalances can have "toxic" effects. Most people can get along just fine on pure oxygen for almost any length of time, although it's seldom used therapeutically since a 30 - 50% O₂ (compared to the ~20% in common air) concentration is sufficient to get as much pO2 saturation of the hemoglobin as is "useful."

John