photosynthesis means green plants convert from carbon dioxide and water and solar energy into carbohydrates and food and oxygen for themselves & if a process converts something to something else, then that process is a source of that something else ->  photosynthesis is a source of oxygen  photosynthesis is a source of oxygen & plants perform photosynthesis ->  plants are a source of oxygen through photosynthesis  plants are a source of oxygen through photosynthesis & if something releases something else then that something is the source of that something else ->  plants release oxygen through photosynthesis  plants release oxygen through photosynthesis & a tree is a kind of plant -> trees release oxygen through photosynthesis