Transition-metal nitrides (TMNs), especially iron-based TMNs, have high electrical conductivity, superior chemical stability and unique oxygen-reaction ability, and have emerged as a new form of noble metal-free electrocatalysts. In this paper, we demonstrate that the ORR-active/OER-inert single function catalyst Fe-N can be made as the OER/ORR-active bifunctional catalyst Co-Fe-N by controlled substitution of Fe with Co. To obtain atomic insights of the Co-enhanced OER catalysis, we develop a new exfoliation method to prepare atomically thin (1.1 nm) nanosheets (NSs) of CoxFe1-xN0.5 (x = 0, 0.05, 0.1, 0.15 and 0.2). Our studies show that both electronic structure and local binding environment of Fe are modified by Co substitution and the Co-Fe-N NSs show a volcano−like OER catalysis with Co0.15Fe0.85N0.5 NSs being the most efficient OER catalyst, showing the lowest overpotential of 266 mV at 10 mA cm−2, a Tafel slope of ∼30 mV dec−1, and excellent stability in the 1.0 M KOH OER condition. Density functional theory (DFT) calculations suggest that the Co-Fe sites in the Co0.15Fe0.85N0.5 structure are optimized to promote OH* to O* conversion, O*-O* coupling and O2 formation. The Co0.15Fe0.85N0.5 NSs are a class of new noble-metal-free catalyst for OER.