Trees don’t actually produce a lot of oxygen, at least not in aggregate. That’s because for every ton of biomass the worlds forests gain through trees growing, you get an equal or larger amount of biomass disappearing through rotting or burning, which… releases CO2 and consumes O2. Only if tree cover as a whole grows can trees in aggregate actually increase atmospheric oxygen and decrease atmospheric CO2.
Unfortunately, that hasn’t happened in centuries, maybe millenia if you discard some minor short-lived recovery periods after major reductions in human population after, for example, Gengis Khan’s conquests in the 13th century, the black plague in Europe in the 14th century or the extinction 90+% of North America’s native population by Eurasian diseases in the 16th century.