Where does the oxygen in our atmosphere come from?

If you are thinking of plants and forests then you are just partially right, in fact, more than 50% of our oxygen comes from the oceans. Phytoplankton are microscopic oceanic organisms that can photosynthesize. They are so abundant that they can be seen from space. The blue stained ocean in the picture shows a "bloom" of phytoplankton. Their massive extent clearly depicts their contribution towards the global oxygen supply.