A comparative study of photodesorption from physisorbed oxygen layers on (highly oriented pyrolytic) graphite and Pt(111) surfaces has been carried out using continuously tuneable synchrotron radiation in the energy range 13 - 35 eV. The photodesorption of both and species is detected. Through monitoring the ion yields as a function of the photon energy and coverage we have proposed a new mechanism for photodesorption of ions induced by inelastic scattering and attachment of photoelectrons originating from the adsorbate layer in the photon energy region between 25 and 30 eV.