There are several known allotropes of oxygen. The most familiar is molecular oxygen (O₂), present at significant levels in Earth's atmosphere and also known as dioxygen or triplet oxygen. Another is the highly reactive ozone (O₃). Others include:

• Atomic oxygen (O₁, a free radical).
• Singlet oxygen  (O₂*), either of two metastable states of molecular oxygen.
• Tetraoxygen (O₄), another metastable form.
• Solid oxygen, existing in six variously colored phases, of which one is and another one metallic.