In combined form it occurs in CO₂, water and oxidised salts. Atmosphere is the direct source of oxygen during respiration of land forms. Aquatic organisms satisfy their oxygen demands from O₂ dissolved in water. During respiration carbohydrate food is oxidised to form CO₂ and water. Burning or combustion also yields CO₂, water, SO₂, NO₂ etc. Microbial oxidation form pool of oxides.

Major replenishment of O₂ occurs through photosynthesis. In the beginning our planet did not possess free oxygen in its atmosphere. It accumulated when photoautotrophs evolved since then it’s level is maintained constant in the atmosphere.