Thermodynamics

The standard state of a pure gas is the pure gas at a pressure of 1 bar and temperature 273 K.

$\Delta_f H^{\circ}_{500}$ is zero for $O_2(g)$.

$\Delta_f H^{\circ}_{298}$ is zero for $O(g)$.

The term 'standard state' implies that the temperature is 0°C.