Manganese oxides, as one of the soil active minerals, are ubiquitous in terrestrial environments. Due to the influence of aging temperature, aging time, and impurity, it inevitably existed defects with different types and concentrations (e. g., oxygen vacancy defect) in manganese oxide, which may affect the fate of heavy metal in environment. Though many literatures have been reported for arsenic removal using manganese oxide, few works focus on the effect of defect in oxidation/adsorption of arsenic on manganese oxides. In this study, manganese oxides with tunable oxygen vacancy defect (OVD) concentrations are prepared by a one-step redox reaction between MnNO₃ and KMnO₄ at different temperature. The result of batch experiment of As(III) oxidation reveals that OVD has considerable effect on As(III) oxidation on manganese oxides. Increasing oxygen vacancy defects concentration in manganese oxide increases the specific As(III) oxidation rate from 0.12 to 0.22 μmol m^-2 min^-1. The origin of the tremendous effect of OVD on As(III) oxidation on manganese oxide is experimentally and theoretically investigated. The results reveal that the presence of OVD in manganese oxide not only makes the adsorption of As(III) on OMS-2 more favorable, but also accelerates the charge transfer from As(III) to Mn atom, thus significantly improving the As(III) oxidation activity. The present work provides a new insight into the effect of OVD in manganese oxide on the fate of arsenic species in environment.