Which process converts oxygen and food into cellular energy?

That process is called as biological oxidation. Glucose is the best example of biological oxidation. Here the end products are same. They are water and carbon bi oxide. In the biological oxidation the energy is released in step wise fashion in multiple steps. In the chemical oxidation the energy is released in single step. The amount of energy is exactly same in both the processes. Chemical oxidation is like jumping from the top of the building. Biological oxidation is like coming down via steps. You come down at the same place. But you may end up in braking your legs in the process of jumping down. In biological oxidation, you do not end up in braking your legs.