when we burn long carbon chains, these are broken up and combined with atmospheric oxygen to form CO2, so any CO2 we produce through burning carbon sources we dig up actually reduces O2 availability. However, a CO2 boom might free up oxygen from other molecules in an indirect way. CO2 dissolved in water forms carbonic acid, this increase in acidity is good for species like Acidithiobacillus ferrooxidans, which will help recover much of the oxygen trapped in complexes like iron oxide.

Or everything goes to hell in a handbasket and earth looks like mars in a few thousand years. Who knows.