How is oxygen in air replaced?

The Oxygen Cycle.

We breath in oxygen

We breth out Carbon dioxide.

Carbon dioxide is a heavy gas and fall to the ground. Whereupon it is absorbed by plants. Whilst in the plant in the process of photosynthesis carbon dioxide is split. The carbon part is retained in the plant as biomass, and the oxygen part is released back into the atmosphere. for humans, animals etc., to breath/consume.

There is also a Carbon Cycle, and a Nitrogen Cycle, where these gases process round through the biOSphere of the Earth.