<p>Oxygen evolution reaction (OER)—2H₂O → O₂ + 4H⁺ + 4e⁻—remains the primary bottleneck in electrochemical water splitting for green hydrogen production. Pentlandite, a bimetallic chalcogenide mineral, has recently shown promise under alkaline conditions, although the elementary processes at the atomic level remain largely unclear. Using density functional theory calculations, we report three generations of pentlandite surface models with varying complexity to decipher the contributions of Fe and Ni sites to OER activity. The first-generation model is based on the pristine pentlandite surface and purports that no OER catalytic activity is observed. The second-generation model takes surface coverage by adsorbed oxygen or hydroxyl into account and suggests that Fe corresponds to the active site in the OER. In contrast, the third-generation model considers not only the surface coverage but also the surface oxidation of pentlandite by exchanging lattice sulfur atoms with oxygen, as observed experimentally. Only this extension shows that both Fe and Ni sites are active centers for OER and that Fe and Ni exhibit distinct limiting steps depending on applied bias, as determined by a degree of span control analysis. Our results demonstrate that when assessing pentlandite with regard to OER, surface oxidation and coverage effects must be explicitly considered in addition to the mechanistic breadth. The reported modeling approach provides the basis for the rational design of next-generation catalysts by highlighting the importance of considering surface oxidation in the theoretical description of energy conversion processes.</p><p></p>