People have a lot of misconceptions about atmospheric oxygen. There is a lot of it. If every photosynthetic organism on the planet died, it would still take ten

People have a lot of misconceptions about atmospheric oxygen. There is a lot of it. If every photosynthetic organism on the planet died, it would still take tens of thousands of years before we had to worry about an appreciable reduction in breathable oxygen levels. We're not going to run out of it unless literally every oxygen-producing organism on the planet dies (trees don't even matter; they could all die and we'd be fine forever). If all plants and phytoplankton die, we'll starve to death and won't have to worry about oxygen levels.