IT is NOT the over saturation of O2. Theres a relative plateau you reach as you hyperventilate in terms of how much O2 you can accumulate (and its pretty close to what you live at normally) but your body is able to blow off a tremendous amount of CO2, which is an acidic compound. As you blow it off your blood becomes more alkaline, which causes some temporary problems-- dizziness, weakness, more anxiety, and carpopedal spasms. If you ever see someone in a BAD panic attack they will sometimes feel cramping in the feet and hands, or even contract the muscles in their feet and hands. It tends to make them panic more and slowing down their breathing is pretty much the only way to turn it around.

Divers actually take advantage of this sometimes, they hyperventilate before a dive to suppress their respiratory drive by blowing off CO2, but it can be kind of a risky technique because your oxygen level doesnt really rise that much.

O2 toxicity tends to only happen with overdelivery of oxygen, on room air its pretty difficult to accomplish.