The study investigates the origin of a 500-fold enhancement in oxygen evolution reaction (OER) activity achieved with mixed (Ni,Fe)oxyhydroxides (Ni1−xFexOOH) compared to their pure Ni and Fe counterparts. Using operando X-ray absorption spectroscopy (XAS) with high energy resolution fluorescence detection (HERFD), it is revealed that Fe3+ in Ni1−xFexOOH occupies octahedral sites with unusually short Fe–O bond distances, induced by edge-sharing with surrounding [NiO6] octahedra. Computational methods confirm that this structural motif results in near optimal adsorption energies of OER intermediates and low overpotentials at Fe sites. In contrast, Ni sites in Ni1−xFexOOH are not active for water oxidation. The findings provide insights into the high OER activity of mixed (Ni,Fe)oxyhydroxides and suggest that Fe substitution can enhance catalytic performance.The study investigates the origin of a 500-fold enhancement in oxygen evolution reaction (OER) activity achieved with mixed (Ni,Fe)oxyhydroxides (Ni1−xFexOOH) compared to their pure Ni and Fe counterparts. Using operando X-ray absorption spectroscopy (XAS) with high energy resolution fluorescence detection (HERFD), it is revealed that Fe3+ in Ni1−xFexOOH occupies octahedral sites with unusually short Fe–O bond distances, induced by edge-sharing with surrounding [NiO6] octahedra. Computational methods confirm that this structural motif results in near optimal adsorption energies of OER intermediates and low overpotentials at Fe sites. In contrast, Ni sites in Ni1−xFexOOH are not active for water oxidation. The findings provide insights into the high OER activity of mixed (Ni,Fe)oxyhydroxides and suggest that Fe substitution can enhance catalytic performance.