I personally would think the bigger issue is keeping the CO2 *inside* the bottle. The partial CO2 pressure inside the bottle is much, much greater than the atmosphere...so that's a big issue.
As far as oxygen, here's my guess. I don't know the exact oxidation reaction(s) that so many beer...
I don't think that's right.
The http://en.wikipedia.org/wiki/Solubility#Solubility_of_gases" of CO2 should only be a function of the temperature and PARTIAL O2 pressure.
By Henry's Law: p_CO2 = constant * [CO2 (aq)]
I'm curious though (and I don't mean this as a rhetorical/jerkish...