MnO2 is widely applied as oxygen reduction reaction (ORR) electrocatalysts in different metal-air batteries (MABs). Enhancing the ORR activity of MnO2 -based catalysts is necessary for improving the performance of MABs. Defect-engineering of catalyst materials is a key approach for enabling the high performance of ORR. Here, the defect-engineering of alpha-MnO2 (211) and beta-MnO2 (110) by oxygen vacancy (O-v) is investigated using the first-principles density functional theory calculation. The geometric structure, adsorption, electronic conductivity, and oxygen reduction reaction (ORR) activity are studied. As a result, the O-v induces the geometric structure that the Mn-Mn and Mn-O distances are closer when the catalysts lose the oxygen atom(s) on the top-layer surfaces. The presence of O-v not only enhances the adsorption energy of *OOH, *O, and *OH, but also increases the electronic conductivity analyzed via the electron transfer. The Bader charge analysis demonstrates that the Mn(IV) can be altered to Mn(III) by the electron accumulation from O-v. The volcano plot of ORR overpotential indicates that having the excess O-v concentration on MnO2 surfaces cannot enhance the ORR activity. The excellent activity is yielded by 12.50 % O-v alpha-(211) and 66.66 % O-v beta-(110) with the ORR overpotential of 0.31 V and 0.60 V, respectively. The results demonstrate that Ov is an essential parameter defining the existence of Mn(III) and Mn(IV) on the surface of MnO2-based catalysts. The optimal ratio of Mn(IV):Mn(III) is in challenge developing the alpha-MnO2 and beta-MnO2 electrocatalyst cathode for metal-air batteries. This study provides a guideline for developing the potential cathode catalyst for MABs, used for harvesting energy. (C) 2022 Elsevier B.V. All rights reserved.