Ozone, o3 (g), is a form of elemental oxygen produced during electrical discharge. is δh∘f for o3 (g) necessarily zero?

I believe that the delta h formation or enthalpy of formation is only zero for pure elements. They are zero because they are the most basic substances, that is you cannot form an element from an element. Since Ozone is already a compound, a mixture of 2 oxygen O2 atoms, therefore the δh∘ f is not necessarily zero.