The oxygen that we currently find in the air has two main sources. The first, and major, source is that oxygen is liberated as a by-product of photosynthetic chemical reactions. The second source occurs in the atmosphere itself when water molecules are split apart after being struck with ultraviolet radiation. This process is called photolysis. The molecule is broken down into one diatomic hydrogen molecule and one free oxygen. The hydrogen molecule is light, and has a quick thermal velocity,so it can escape Earth's atmosphere leaving behind the free oxygen to form a diatomic oxygen molecule. The main source of this atmospheric water was volcanic eruptions and outgassing.

Basically before photosynthesis began on large scales, free oxygen molecules in the atmosphere were drawn down in oxidation reactions with free metal ions on the Earth's surface, mainly iron. Even when photosynthetic organisms were taking control of the atmosphere, the free oxygen molecules were being placed into the rock record in oxidation reactions. This forms red colored rocks, like you find in the southwestern United States. Because of this, early Earth had a reducing atmosphere, composed mainly of methane,carbon dioxide, water, carbon monoxide, and nitrogen. This would be a very hostile place to most modern forms of life.

The Earth is unusual among planets in that it has as abundance of oxygen. However, this essential element was not plentiful on our planet until microorganisms known as cyanobacteria began to produce it by photosynthesis.