Rock-salt-type MnO is rarely used as an electrocatalyst because of its relatively poor activity. Herein, through an in situ preparation and Ca-substitution of MnO/C, we were able to obtain a novel composite, i.e., Ca_0.5Mn_0.5O/C, as a highly active, stable, and cost-effective oxygen reduction reaction (ORR) catalyst in alkaline media. Ca_0.5Mn_0.5O/C and MnO/C share a similar rock-salt phase. In comparison to MnO/C, Ca_0.5Mn_0.5O/C follows a more effective four-electron pathway (versus a two-electron pathway) and displays higher ORR activity, including a more positive onset potential (by 0.05 V), a more positive half-wave potential (by 0.04 V), and a higher current density (by 1.48 mA cm⁻²). The Ca_0.5Mn_0.5O/C also shows comparable mass activity, higher activity per material cost, and superior stability in alkaline media in comparison to commercial Pt/C. Additionally, the as-prepared Ca_0.5Mn_0.5O/C exhibits higher ORR activity than the physical mixture of Ca_0.5Mn_0.5O and carbon. The enhanced ORR performance of Ca_0.5Mn_0.5O/C is likely due to (1) the presence of the divalent redox pair Mn^II/Mn^III; (2) the formation of MnOOH on the Ca_0.5Mn_0.5O surface; and (3) a stronger synergistic interaction between Ca_0.5Mn_0.5O and C resulting from the in situ preparation method. This work provides new routes to develop advanced electrocatalysts using transition-metal-oxide/carbon composites.