if a process converts something into something else, then that process is a source of that something else & photosynthesis means green plants convert from carbon dioxide and water and solar energy into carbohydrates and food and oxygen for themselves ->  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 & a source of something produces that something ->  plants produce oxygen through photosynthesis  plants produce oxygen through photosynthesis & if something is given off of something , then something is the product of something -> plants give off oxygen through photosynthesis