Where does oxygen come from?

oxygen is from carbon dioxide. This is done by plants.they are the only ones tat take in co2 and release o2. this process is called photosynthesis. the carbn dioxide is taken in and the varous plant cells use this for chemical procceses in their cells using various pathways that take only the carbom atom and release the o2 as a waste product. co2 comes in from he stomata of their leaves and o2 goes out via the same ute but as a waste product.thats why trees are so important.:P

It came from the plants,,

We know that oxygen definitely comes from photosynthesis and rain forests do produce a significant but decreasing amount because of all the rainforest destruction.