To explore metal oxide oxide support interactions and their effects, the mechanism of CO oxidation on PdO catalysts with rutile TiO2 or TiO2-x (TiO2 with a bridging oxygen vacancy) as the support, was studied by density functional theory calculations, compared with that on pure PdO surface. For TiO2 as the support, support effect leads to the change of the preferential CO adsorption sites from the coordinatively unsaturated Pd (Pd-cus) site on pure PdO surface to the bridging site of coordinatively unsaturated Pd and O atoms (Pdcus and O-cus), thus altering the reaction pathway of CO oxidation, whereas the support effect has little influence on the energy barrier. However, for TiO2, as the support, the presence of the oxygen vacancy leads to the energy barrier remarkably decreased compared with that on pure or TiO2-supported PdO surface. The change of Bader charges indicates the oxygen vacancy in the support can tune the oxidizability of PdO surface active oxygen Ocus, thus adjusting the CO adsorption strength at the bridging site of Pd. and O-cus to be favorable for the extraction process of O-cus. Tuning oxygen vacancies in supports can be used as a new perspective to design improved supported oxide catalysts. (C) 2017 Elsevier B.V. All rights reserved.