Oxygen (O) is responsible for cellular respiration in plants. Plants acquire O by breaking down carbon dioxide (CO2) during photosynthesis and end up releasing the majority of it as an unnecessary byproduct, saving a small portion for future energy.
Most of the O plants take in is expelled as a byproduct. Only a very small amount is actually used by the plant for respiration.
Plants don't absorb O from the air, but instead acquire it during the breakdown of carbon dioxide (CO2) as part of photosynthesis.
Oxygen interacts with nitrogen (N) in a process called denitrification, and it affects other elements' oxidation states as well.