Oxygen and ozone. Computer artwork of oxygen (O2) and ozone (O3) molecules in the atmosphere above the Earth. Ozone is an allotrope of oxygen, and is made up of three oxygen atoms (blue spheres). It is produced in the stratosphere by the action of high-energy ultraviolet (UV) radiation on oxygen. It forms a stable layer, known as the ozone layer, which screens the Earth from harmful UV radiation.