As a model of Au-titania core-shell catalysts, an ultrathin rutile TiO2(110) layer inversely supported on Au(112) has been examined by DFT. O2 adsorbs and activates on the pentacoordinate Ti site of rutile(110) as O2 π* orbitals accept electronic charge from the Au support via the oxide, as was found for Au/TiO2. O2 reacts with CO to yield CO2 and O, which then reacts with CO to yield CO2 (barrier ~0.6eV). O2 adsorption weakens with increasing O2 coverage and oxide thickness. The reactivity of O2 changes negligibly even if we replace Au with Ag.