Not sure you're getting what parent said. Oxygen is an element. It doesn't get "used up" in the sense that you have less. It simply gets bound in different form

Not sure you're getting what parent said. Oxygen is an element. It doesn't get "used up" in the sense that you have less. It simply gets bound in different forms (like more complex molecules). It just takes energy to release it so we can breathe it again. No loss (unless you run it through a fusion reactor). Photosynthesis is one such process.