Fuel cells are considered promising next-generation energy conversion devices because of their high efficiency and environment-friendly attributes. However, a large overpotential for the oxygen reduction reaction (ORR) at the cathode minimizes the energy efficiency of these devices. In this context, ORR catalysts based on 3d transition metal oxides have attracted significant attention as affordable and highly active catalysts with potential to replace expensive noble metal catalysts (e.g., Pt). In this study, the ORR catalytic activities of ‘oxygen-excess’ perovskite La1−xSrxMnO3+δ (x = 0, 0.1, 0.2, 0.3, and 0.4), with both higher activity and stability compared to a regular perovskite, i.e., La0.8Sr0.2MnO3, were investigated and the performances were evaluated with respect to changes in the Mn valence number, amount of cation vacancies and Mn-O bond length. Sr substitution can affect the formation of Mn4+ ions and modulate the cation vacancies, thus changing the length of the Mn-O bond. The lowest overpotential was observed for La0.8Sr0.2MnO3+δ, where the Mn-O bond length was the shortest, resulting in the highest activity. Overall, this work provides guidelines to improve the ORR catalytic activity of Mn-based perovskites.