(i) In the atmosphere, oxygen is found in the elemental form to the extent of 21%.
(ii) In combined form, it occurs as CO2 both in the earth’s crust as well as ih the air.
(iii) In the crust, it is found as the oxides of most metals and silicon, and also as carbonate, sulphate, nitrate and other minerals.
(iv) It is also an essential component of most biological molecules like carbohydrates, proteins, nucleic acids and fats (or lipids).