Hey, it appears you don't quite understand how plants function. Plants indeed reduce oxygen to H2O to make ATP, the energy chemical all living things must produce. They do it with the same enzyme, ATP synthase, that every living organism on earth possesses. They do this by oxidizing carbohydrates into CO2 through the same reactios we do.

What plants are also able to do is harvest the energy in sunlight to turn CO2 and water back into carbohydrates. Because most of their structure is made of carbohydrates and carbohydrate compounds, they must do a lot of this, so they net consume CO2 and produce O2.