Amazon
The Amazon rain forest produces more than 20% the world's oxygen supply. The Amazon River pushes so much water into the Atlantic Ocean that, more than one hundred miles at sea off the mouth of the river, one can dip fresh water out of the ocean. The volume of water in the Amazon River is greater than the next eight largest rivers in the world combined and three times the flow of all rivers in the United States .

? Since Algae in the oceans produce more then 80% of the oxygen supply how can the Amazon produce over 20%?
Over 20% of the land-based O2 I'd go for.