Oxygen is a major component of all living organisms. All the organisms and plants required oxygen for aerobic respiration which they absorb from environment. Only plants release oxygen into environment during photosynthesis. Earth atmosphere contains 20.99% of O_2.

Thus oxygen cycle is the biogeochemical cycle that describes the movement of oxygen within and between its three main reservoirs- The biosphere (living) the atmosphere (air) and the lithosphere (earth crust).  This cycle includes:

(i)        The main driving factor for oxygen cycle is photosynthesis, which is responsible for modern earth's atmosphere and life. This process produces oxygen and sugar by utilizing CO₂ and water in presence of sunlight and chlorophyll.

                        Sunlight

6CO₂+6H₂O                 C₆H₁₂O₆+6O₂

(ii)      Oxygen also enters into atmosphere by process of photolysis in which high energy U.V radiations breaks down atmosphere water and nitrous oxide back into its component atoms.

              _U.Vrays

2H₂O              4H+O₂

               _U.V.rays

2H₂O               4H+O₂

(iii)    Oxygen is mainly consumed from atmosphere via respiration and natural decay mechanism in which animals, plants and bacteria consume oxygen and release carbon dioxide.

                        _respiration

C₆H₁₂O₆+6O₂             6CO₂+6H₂O

(iv)             Some of oxygen is also consumed in natural oxidation of various mineral on earth crust.

Oxygen is also cycle between biosphere and lithosphere in which marine organisms play important role. Marine organisms have calcium carbonate sheet. That after death is deposited on sea floor and forms limestone rock in the lithosphere.