In this paper, we report the study of adsorption and dissociation of oxygen on a polycrystalline Ag surface by means of optical second-harmonic generation (SHG) in a Ultra-high vacuum. From the experimental results, we find that the nonlinear surface currents is affected by adsorption of oxygen, and this is reflected on tne change of parameters a and b. A simple model is used to fit the experimental results and is in good areement with the experiment.