[jome]

It's not just high co2 that triggers contractions/breathing reflex. Like Veronika suggests, it's ultimately more complicated than that. I believe that the low o2 receptors (or what ever, chemoreceptors?) only activate when the oxygen level reaches a pretty low level, so with pure O2 they never activate and you get these kind of half-contractions or whatever...But on a normal static, for the last minute or so, you feel the combined effect of high co2 and low o2. Towards the end of a maximum static I get what I call "hypoxic contractions" (for the lack of a better term), which compared to the co2 ones are much uglier and ultimately determine the break point. The CO2 contractions you can learn to ignore and tolerate pretty well...

I haven't tried my self, but having talked with some people who have tried pure o2, they describe it as something like "you get contractions, but they just don't feel bad"...