Not all organisms breath oxygen though (although aquatic animals actually do breath the oxygen in water and not water itself). Plants breath sunlight (and eat carbon dioxide), while a variety of bacteria breath exotic substances like sulphate (SO4--), hexvalent uranium (UO2++), ferric iron (Fe+++, aka rust), or arsenate (AsO4---). Dioxygen is used by the majority of atmospheric organisms (exist in air) because it is high energy (reactive) and has an energy barrier (like pushing a rock up a small hill to knock it down a cliff) preventing it from reacting immediately and explosively (like dichlorine does).