The electronic properties of a single Ru atom and a Ru10 cluster adsorbed on stoichiometric and reduced anatase, a-TiO2(101), and tetragonal zirconia, t-ZrO2(101), surfaces have been determined with density functional theory calculations with Hubbard corrections (DFT+U). The main purpose of the work is to better understand the role of dispersed metals on the surface of titania and zirconia catalysts in conversion of biomass to biofuels. On the stoichiometric surfaces, the metal adsorption does not imply major charge transfers. The situation is different on the reduced surfaces where electron transfer occurs from the oxide to the metal; this effect is more pronounced on zirconia than on titania. On both surfaces, the presence of the Ru nanoparticle favors the removal of O from the surface layers. This can result in the occurrence of O reverse spillover, with displacement of an O atom from a lattice position of the stoichiometric surfaces to a specific adsorption site of Ru10. This process is thermodynamically accessible for both TiO2 and ZrO2 surfaces showing that the metal deposition can result in an easier reduction of the oxide support due to this effect more than to a direct electron transfer from the metal.