I didn't notice any replies mentioning that extra oxygen reduces your body's breathing reflex.
One of the factors in sleep apnea incidents is your breathing reflex. It's hypercapnia (the buildup of CO2) that eventually forces your body to overcome whatever is causing the apnea/hypopnea in the first place.
For example, If it's a simple breathing obstruction, for example your palate collapsing when you relax during sleep, then the CO2 buildup can eventually either stress you enough to un-relax it, or cause your body to try hard enough to overcome it. If it's a neurological or other factor causing you to simply stop trying to breathe (for example, unobstructed apnea, happens in a smaller proportion but still many people), the CO2 buildup can overcome this and cause you to resume breathing.
When you stop or slow breathing for whatever reason, you don't instantly run out of oxygen. The left-over oxygen in your lungs is known as your FRC, Functional Residual Capacity. With normal lungs full of air, you are thought to have something like two minutes' worth, normally, though the CO2 buildup happens gradually during that time and your breathing reflex increases pretty quickly. If you have the usual end-expiratory apnea (you breath out, then stop breathing) it's worse, of course, because you have less air in your lungs, but my point is the same:
Supplementing oxygen increases your FRC by increasing the extra oxygen in your lungs. This, once it reaches a significant amount, reduces your body's breathing reflex, and/or retards the timing of its response.
I suppose one could argue that this may render oxygen use (at least by some) irrelevant or worse. They will have more FRC, which will protect them from the harm of the apnea, but may also make the apnea incidents last longer, the CO2 buildup triggering forced breathing being delayed. But perhaps prolonged incidents from oxygen use, themselves, would be worse. The cortisol buildup from stress may be increased simply because of the longer timespan, for example.
I am not offering a conclusion either way, regarding O2 supplementation. But its impact on the breathing is a factor worth considering.
