I got the idea from noticing that the CO2 level can make it harder for me to think, so I wondered what would happen if I went in the other direction (more oxygen).
Buy a $20 SpO2 monitor, and realize that your blood is already saturated with O2. You'll find that you're generally between 97%-99% to begin with, so what is adding oxygen going to do? The amount of oxygen that reaches your tissues is a function of the carrying capacity of your erythrocytes, and unless you're very sick or in an extreme environment, that isn't driven by ambient O2 levels.