The exploration of efficient, stable, and inexpensive oxygen reduction reaction (ORR) electrocatalysts to replace Pt/C in fuel cells and metal–air batteries still remains an ongoing challenge. Herein, we report an efficient ORR electrocatalyst composed of single-atom FeNx and γ-Fe2O3 nanoparticle co-functionalized hollow graphitic carbon nanofibers (FeNx/Fe2O3-CNFs). It is found that the FeNx species serve as the active sites, while the γ-Fe2O3 nanocrystals function as a co-catalyst to boost the ORR catalytic activity. The obtained FeNx/Fe2O3-CNFs exhibit desirable ORR electrocatalytic activity with a 4-electron transfer pathway, a half-wave potential of 0.81 V approaching that of commercial Pt/C, low hydrogen peroxide yields (<6% at 0.2–0.7 V), long-term stability (87.14% after 30 000 s), and excellent methanol tolerance. The assembled Zn–air battery based on the FeNx/Fe2O3-CNFs has an open circuit voltage of 1.51 V and superior energy density of 920 W h kg−1. This work highlights the significant contribution of the co-catalyst in electrocatalysis.