Earth, and the rest which largest is Plankton.
As we know plankton are any drifting organisms which live in oceans, seas, and bodies of fresh water, become the most important organism in the world because it's being a food for aquatic life.
Plankton consists of the remains of marine animals and plants. Although including living beings, plankton has no strength to fight the flow, that's why it always carried by the tide and wind carry them away.
And after I've found out from which the small creatures as it could be the largest producers of oxygen, it's because of phytoplankton photosynthesis that produces lots of oxygen and released into the air that meets the Earth's atmosphere. No wonder if it became the largest producer of oxygen, because plankton are scattered across the waters on this earth. Even such a small creature like this can be very beneficial for the whole life in this earth. =)