Somebody with a better chemistry background is going to have to field this one. But in my mind it's still the purists co2 we can get and if it was a major problem bottle conditioning would more detrimental to the beer.Not a chemist here. But I believe it has more to do with its not oxygen.
CO2 is the primary gas produced by fermentation. However, it is of course far from the only gas emitted. There's also various sulfur compounds, aromatic esters, alcohols, water vapor, etc. You are correct -- none of these are oxygen. The only way to produce oxygen that I know of would be via electrolysis by running an electrical current through the water like a battery, and I seriously doubt any brewer is doing that! Nevermind the flammable hydrogen gas that would be produced along with it!
What about things like sulfur dioxide coming from the fermentation? Is it possible it could break down and release O2?