What is the importance of oxygen in the atmosphere?

Without oxygen, ##"O"_2##, there is no animal life that requires oxygen.

In order for life to survive on earth oxygen is needed for those which aerobically respire. It is used in respiration which is converting sugar (glucose) ##"C"_6"H"_12"O"_6## using oxygen, ##"O"_2## into carbon dioxide, ##"CO"_2## and water ##"H"_2"O"## this reaction gives energy needed for cells to operate. Since plants require ##"H"_2"O"##, ##"CO"_2##, and sunlight for photosynthesis, they would survive and eventually make oxygen as a by-product of photosynthesis.