We reinvestigated non-dissociative chemisorption of O-2 on Ag(001) by the King and Wells reflectivity method and high-resolution electron energy loss spectroscopy over a wide range of impact energies of O-2. In analogy to O-2/Ag(110), we find that the sticking probability has a maximum at a total energy of 0.7 eV corresponding to the onset of direct inelastic scattering from the repulsive part of the chemisorption potential. Below it, normal energy scaling is approximately obeyed, whereas above it, a breakdown of such behaviour is observed. The decrease in sticking probability at high impact energy is more pronounced for O-2/Ag(001) than for O-2/Ag(110).