http://wtamu.edu/~cbaird/sq/2013/01/05/how-do-trees-give-earth-all-its-oxygen/
All of earth's oxygen does not come from trees. Rather, the atmospheric oxygen that we depend on as humans comes predominantly from the ocean. According to National Geographic, about 70% of the oxygen in the atmosphere comes from marine plants and plant-like organisms. These ocean-living plants release molecular oxygen as a waste product of photosynthesis (as do most plants). In photosynthesis, plants capture sunlight and use its energy to split carbon dioxide and water, making sugar for itself and releasing oxygen as a by-product. The dominance of ocean life as earth's top oxygen producer makes sense when you consider that the majority of the earth is covered with ocean.
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