Oxygen occurs as an elementary substance in two allotropic varieties: the natural oxygen (second constituent of the air) and the ozone. It also forms a great variety of compounds and, in the combined state, it is the most abundant element of the terrestrial crust (in the silica and silicates form) and of the oceans (in water).
Oxygen reacts with some complex transition metals which act as transporters of the substance, i.e., they capture and give O2 in a reversible way. Some of these complex metals constitute prosthetic groups of indispensable proteins to the metabolic process of breathing. That is the case of the hemoglobin and of certain hemocyamines that are natural carriers of oxygen.