The thermally induced oxygen vacancies present across the intra/inter-crystalline sites and surface of ultrafine CoMn2O4 (CMO) electrodes ameliorate electrochemical performance of Li-O2 batteries (LOBs). Oxygen deficient CMOs are synthesized via a two-step process: in situ reduction to achieve a large surface area of 151.3 m2 g−1 and thermal treatment at 400 °C in pure Ar. The oxygen deficient CMO electrode presents a higher initial capacity, lower overpotential, better cyclic stability, higher Coulombic efficiencies and higher rate capabilities than the as-prepared CMO electrode without heat treatment. While the CMO electrode presents an excellent catalytic behavior in oxygen reduction reaction (ORR), the oxygen vacancies mitigate the migration of Li+ ions and electrons and act as active sites for O2 in the oxygen evolution reaction (OER). The ex situ characterization also proves a lower kinetic charge transfer resistance and higher catalytic activities of the oxygen deficient CMO electrodes in the decomposition of discharge products during the discharge/charge cycles.