It seems like an odd thing to be concerned about, given the fairly tight link between the plant activity that laid down today’s fossil fuels(especially in the boom years when plants had developed synthetic polymers like cellulose and lignin; but nobody had yet evolved a way to crack them) and the plant activity that gave Earth an oxygenated atmosphere(which it basically didn’t have until plants got started, broke through the buffer provided by oxidation of iron and such in the crust, and continued emitting until anaerobic ecosystems retreated into various niches).
To reverse that process and get back to a non-oxygen atmosphere you’d more or less need to get all the plants, minus whatever margin of error the period of geological oxidation provides you with; and you’d need to do it fast and hard enough to prevent contemporary plants from photosynthesizing abundantly.
Also an odd thing to be concerned about, given that the amounts of CO2 required to produce more or less apocalyptic greenhouse effects can be reached well before you change the partial pressure of oxygen enough to cause direct problems for all but the sickest or most geographically extreme populations. Especially with modern air travel we can directly observe the changes in altitude, and correspondingly partial pressure, that people typically don’t even notice.
