20.94% oxygen is found in atmosphere. Diatomic O (oxygen) and triatomic O (ozone) are two compounds of oxygen found in nature.
Processes in which oxygen is used up (i) Respiration in most organisms.
(ii) Combustion of fossil fuels.
(iii) Formation of oxides, e.g. nitrogen oxides during atmospheric fixation of nitrogen.
(iv) Decomposition of organic matter.