The oxygen reduction reaction (ORR) and diffusion mechanisms on 25% Sr-doped LaMnO3 (LSM) cathode materials as well as their kinetic behavior have been studied by using spin-polarized density functional theory (DFT) calculations. Bader charge and frequency analyses were carried out to identify the oxidation state of adsorbed oxygen species. DFT and molecular dynamics (MD) results show that the fast O2 adsorption/reduction process via superoxide and peroxide intermediates is energetically favorable on the Mn site rather than on the Sr site. Furthermore, the higher adsorption energies on the Mn site of the (110) surface compared to those on the (100) surface imply that the former is more efficient for O2 reduction. Significantly, we predict that oxygen vacancies enhance O2 reduction kinetics and that the O-ion migration through the bulk is dominant over that on the surface of the LSM cathode.