It's quite simple, any live cells need oxygen, it's the base element to all life on the planet.
Human obtain oxygen by breating it. The lungs absorb the oxygen then transfers it to the blood cells which pass the oxygen throughout the body, Flatworms absorm the oxygen in the water. Water is 2 parts hydrogen and 1 part Oxygen. Plants as well as animals breath all the time taking in O2 and breathing out CO2. In photosynthesis there is a dark phase where they are not taking in CO2 and combinig it with H2O to make glucose, In the light phase they are taking in CO2 and H2O