Recent experiments have demonstrated that oxygen reduction reaction (ORR) can contribute significantly to Mg corrosion in addition to hydrogen reduction reaction (HER). However, its contributions at various stages of Mg corrosion remains unclear. Using DFT calculations, we investigate ORR and HER on both pristine and Fe/Mn alloyed Mg, MgO and Mg(OH)₂ during Mg corrosion. We found that both HER and ORR on pristine and alloyed Mg contribute to the overall cathodic current density: HER contributes much more significantly than ORR, while the ORR contribution is up to the mass transport limit and would diminish over time. In addition, we found that ORR activities on Fe/Mn alloyed MgO are high, suggesting an alternate source of cathodic ORR contribution during the whole course of Mg corrosion. Excellent agreements are found between our findings and a recent experiment.