We demonstrate by vibrational spectroscopy that open steps are the active site for O2 dissociation on Ag(410), and that the barrier to adsorption at defects can be measured by energy and angle resolved investigation of the dynamics of the gas-surface interaction. We identify a molecular adsorption channel with considerably reduced activation barrier and a nonactivated dissociative pathway. The O2 sticking probability is increased at steps and strongly reduced at terraces with respect to Ag(100), implying that reactivity is influenced by terrace width.