Answer:
Atmospheric oxygen is a result of photosynthesis by plants (including cyanobacteria - blue -green algae - which was responsible for the initial conversion of earth 's atmosphere from its anoxic state) .
Photosynthesis is carried out in plants. It employs energy from the sun and naturally occurring water and carbon dioxide to produce sugar and oxygen gas. The oxygen is released to the atmosphere, and the sugar is formed into plant matter, which it eventually and is buried and turned into coal, oil and gas.