In this paper a theoretical study of polarization properties of a silver nanosphere touching a homogeneous silver substrate and covered by oxide layers of increasing thickness, is reported. Oxide layers are often deposited on metallic nanostructures in metal-enhanced fluorescence (MEF) or surface-enhanced raman scattering experiments to avoid nonradiative energy transfer from emitters to the metal, and to increase the nanoparticles stability against thermal processes and laser exposure. Not much has been said on the effect of the oxide on the field enhancement of such kind of plasmonic systems. This work aims at filling this gap by shedding light on the effects of the oxide coverage on the near and far field behavior: numerical simulations performed in the framework of the discrete dipole approximation show the presence of new resonances in the absorption spectra and, of major importance for MEF applications, a strong enhancement of the near field around the nanosphere.