Sometimes it's part of sugar. Ok, we still need to find some way that eating all this sugar and burning it up with oxygen is not going to use up all the oxygen. Get ready for this. When plants make sugar they release back into the air all that oxygen that originally combined with the sugar when it burned. So we have a nifty loop here.
Plants make sugar from water, carbon dioxide and sunlight.
We combine sugar with oxygen and release the energy from the sunlight and release more water and carbon dioxide back into the air for the plants to make more sugar. Notice that the same materials are recycled over and over and the only thing we really take out of the process is the energy from the sun. That's why we are all solar powered. We use sugar from the plants as a way of getting that solar energy into our bodies. The only thing that actually gets used up is sunlight, everything else is recycled.