Oxygen is vital to living organisms and when it is deficient, portable oxygen units are used to administer oxygen to the patient. In the same way we humans breathe out carbon dioxide, plants absorb it and need it to survive. Humans need oxygen for their cells to keep the body healthy.
Without it, food could not release the energy that a body needs to keep living, and for the carbon dioxide, vital for plants but harmful to humans, to be released. In fact, if we do not exercise sufficiently we starve our bodies of oxygen and this is one reason why couch potatoes become sickly. Only planets with oxygen can support life as we know it.
