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It's not so much that O2 is an exception as it is that the standard enthalpy of formation of an element in its most stable form is 0 (this definition is found on page 294 of the book). I think this makes more sense if you think about the "reaction" you'd need to form these elements. You could write down a reaction, but since the reactants of the reaction would be the constituent elements in their most stable form, the reactants and products of a reaction making an element in its most stable form would be the same.
The enthalpy of formation of oxygen (O2) is 0 because O2 is the element oxygen in its most stable state. Therefore, elements in their most stable states (e.g., H2 for hydrogen, F2 for fluorine, C for carbon, etc.) all have enthalpy of formation equal to 0. In all other less stable forms, enthalpy of formation will not equal 0.
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