One a friend and I hooked ourselves up to continuous pulse oximetry and had a contest to get the lowest recorded oxygen level. We tried everything we could think of, from just holding our breath to end-expiratory breath holding to hyperventilating to clear O2 (I used to do some recreational free-diving) beforehand to exercising (jumping jacks)...

Neither of us could get it below 98%, and this was at a mile of elevation (UNMH in Albuquerque).

If you've ever done the wim hof breathing method, it is a very intense experience.

Basically hyperventilation + long breath holds. Probably similar to what free divers do without the mammalian dive reflex due to the cold water. Or like a dangerous game kids used to do when I was in school where you hyperventilate and then have someone press on your chest until you pass out.

But anyway, I'm not sure if the science would back it up, but Wim Hof describes it as over oxygenating the blood and then stopping and letting CO2 ramp up or something. Whether it is significantly dropping the CO2 or increasing oxygen during the hyperventilation phase, isn't it kind of the same thing? Adjusting the ratio.

Anecdotally, when I was doing it regularly I seemed to not get sick at all.

No it would stay at pretty much 100% (as is normal). But co2 goes down, which lessens the ability of oxygen to come out of the blood. That's why you get dizzy when hyperventilating

Blood oxygen saturation is always near 100% in a healthy person. 95% is the low end of normal. Dropping to 90% is considered hypoxemia, and 80% is a medical emergency. So there really shouldn’t be any room to increase it significantly.

Nope. The blood is already fully saturated with oxygen (in a healthy-ish person) at rest. Even intensely breathing pure o2 can't give you a saturation higher than 100%.