Cubic perovskite LaMnO3 surface models were constructed to elucidate the mechanism of oxygen reduction using quantum chemical calculations with molecular dynamics (MD) simulations. Calculations predict that both dissociative and molecular adsorption may occur, depending on adsorbate configurations. Superoxo- or peroxo-like species may locate at La, Mn, and Osub active sites with different vibrational frequencies and atomic charges. A stepwise elementary reaction sequence via the superoxo- or peroxo-like intermediates at both perfect and defective LaMnO3 was constructed by mapping out minimum-energy paths (MEPs) using the nudged elastic band (NEB) method. Charge transfer for the O2-LaMnO3 interactions was also explored by Bader charge analysis. In particular, ab initio MD simulations carried out to simulate solid oxide fuel cell conditions at 1073 K suggest that oxygen vacancies enhance O2 dissociation kinetics.