The 3d transition metals have been investigated as active centers in NiOOH to catalyze oxygen evolution reaction (OER); however, questions about the mechanism remain. Here, we study how cobalt (Co) and iron (Fe) doping and CoFe codoping determine the OER activity of NiOOH via experiments and theoretical calculations. The results show that both Co and Fe, with enhanced density of states near the Fermi level, decrease the overpotential by increasing the binding energy of O∗ and consequently exhibit higher activities than nickel. In particular, Fe, with nearly optimal O∗ binding energy, exhibits the lowest overpotential of 181 mV to reach 50 mA cm−2. In the case of CoFe codoping, Co alters the electronic states of Fe, which weakens the Fe-OOH bond and slightly increases the overpotential. Based on the calculated activities, an overpotential contour plot is constructed, providing guidance for catalyst design via modulating electronic structures and intermediate binding energies.