Manipulating the spin ordering of the oxygen evolution reaction (OER) catalysts through magnetization has recently emerged as a promising strategy to enhance performance. Despite numerous experiments elaborating on the spin magnetic effect for improved OER, the origin of this phenomenon remains largely unexplored, primarily due to the difficulty in directly distinguishing the spin states of electrocatalysts during chemical reactions at the atomic level. X-ray emission spectroscopy (XES), which provides information sensitive to the spin states of specific elements in a complex, may serve as a promising technique to differentiate the onset of OER catalytic activities from the influence of spin states. In this work, we employ the in situ XES technique, along with X-ray absorption spectroscopy (XAS), to investigate the interplay between atomic/electronic structures, spin states, and OER catalytic activities of the CoFe₂O₄ (CFO) catalyst under an external magnetic field. This enhancement is due to the spin magnetic effect that facilitates spin-selective electron transfer from adsorbed OH^- reactants, which strongly depends on the spin configurations of the tetrahedral-(T_d) and octahedral-(O_h) sites of both Fe and Co ions. Our result contributes to a comprehensive understanding of magnetic field-assisted electrocatalysis at the atomic level and paves the way for designing highly efficient OER catalysts.