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So O2 has zero of heat of formation (or enthalpy of formation). The enthalpy of a reaction is defined like this: H = Hproducts-H reactants. The enthalpy of reaction in this case, is the same as the heat of combustion. Hproducts is obtained summing the heats of formation.
1 Standard Heat (Enthalpy) of Formation, ÎHfo, of any compound is the enthalpy change of the reaction by which it is formed from its elements, reactants and products all being in a given standard state.  By definition, the standard enthalpy (heat) of formation of an element in its standard state is zero, ÎHfo = 0.