In this paper, we report the discussion on the nonlinear optical properties of an oxygen-covered Ag as well as the experimental studies with optical secondharmonic generation (SHG) in an ultrahigh vacuum system. The resulted SH signals are found to be very sensitive to the charge transfer between oxygen and silver. Based on the SHG measurement of oxygen adsorption dynamics, we seggest that the unmber of electrons donated to silver from the π-filled orbital might be more than that transfered from silver to the π* anti-bonding orbital of adsorbed oxygen molecules. Besides, the temperature programmed dissociation of adsorbed oxygen molecules was investigated, which could be explained in the framework of a simple model.