I will note that I have survived over 70 years of breathing 20% (and sometimes higher O2). Only noticable problems have been at higher elevations where I had less of it.
I believe the first hints that free radicals theory doesn't quite work was when they tried feeding huge amounts of anti-oxidants to competitive athletes expecting their performance to improve by leaps and bounds. Strangely enough, that didn't happen.
Of course the flip side is people living at higher elevations are believed to have longer average lifespans.