Breathing oxygen as opposed to what? I wonder if it has anything to do with the air being clean. Perhaps fresh air would have a similar effect. Fresh air is getting harder to find these days

Hyperbaric oxygen therapy involves breathing pure oxygen in a sealed, pressurised chamber

As opposed to normal atmospheric pressure air which also includes CO2 and nitrogen. Some athletes and celebs use Hyperbaric O2 tents to prepare for... who knows what.

we get very little oxygen around here, and many headaches.