This study investigates the identification of highly active Fe sites in (Ni,Fe)OOH for electrocatalytic water splitting. Using operando X-ray absorption spectroscopy (XAS) with high energy resolution fluorescence detection (HERFD), the research reveals that Fe³+ in (Ni,Fe)OOH occupies octahedral sites with unusually short Fe–O bond distances, induced by edge-sharing with surrounding [NiO₆] octahedra. Computational methods show that this structural motif results in near optimal adsorption energies of OER intermediates and low overpotentials at Fe sites. In contrast, Ni sites in (Ni,Fe)OOH are not active for water oxidation. The study identifies that Fe³+ cations substitute for Ni³+ in the lattice of γ-NiOOH, leading to a significant reduction in OER overpotential. The Fe–O bond distance in (Ni,Fe)OOH is 6% shorter than that in γ-FeOOH. DFT+U calculations confirm that Fe³+ cations in γ-NiOOH exhibit significantly lower overpotential than Ni³+ cations. The results show that Fe sites in (Ni,Fe)OOH achieve nearly optimal adsorption energies, contributing to the high OER activity. The study concludes that Fe³+ cations in (Ni,Fe)OOH are the active sites for the OER, and that the substitution of Fe into NiOOH enhances the OER activity. The findings provide insights into the structure and electronic properties of (Ni,Fe)OOH catalysts and their role in water splitting.This study investigates the identification of highly active Fe sites in (Ni,Fe)OOH for electrocatalytic water splitting. Using operando X-ray absorption spectroscopy (XAS) with high energy resolution fluorescence detection (HERFD), the research reveals that Fe³+ in (Ni,Fe)OOH occupies octahedral sites with unusually short Fe–O bond distances, induced by edge-sharing with surrounding [NiO₆] octahedra. Computational methods show that this structural motif results in near optimal adsorption energies of OER intermediates and low overpotentials at Fe sites. In contrast, Ni sites in (Ni,Fe)OOH are not active for water oxidation. The study identifies that Fe³+ cations substitute for Ni³+ in the lattice of γ-NiOOH, leading to a significant reduction in OER overpotential. The Fe–O bond distance in (Ni,Fe)OOH is 6% shorter than that in γ-FeOOH. DFT+U calculations confirm that Fe³+ cations in γ-NiOOH exhibit significantly lower overpotential than Ni³+ cations. The results show that Fe sites in (Ni,Fe)OOH achieve nearly optimal adsorption energies, contributing to the high OER activity. The study concludes that Fe³+ cations in (Ni,Fe)OOH are the active sites for the OER, and that the substitution of Fe into NiOOH enhances the OER activity. The findings provide insights into the structure and electronic properties of (Ni,Fe)OOH catalysts and their role in water splitting.