Transition metal cobalt (oxy)hydroxides are promising catalyst candidates for the oxygen evolution reaction (OER). Their activity can be significantly enhanced by doping chromium; however, the optimal catalyst elemental composition is unknown, and the exact role of Cr remains elusive. Here, density functional theory (DFT) calculations and comprehensive catalyst characterization are combined to address these two questions. Guided by DFT calculations, a series of amorphous ternary Co‐iron(Fe)‐Cr (oxy)hydroxides with systematically tuned elemental compositions are synthesized. Performance atlases of these catalysts are established based on different experimental descriptors, which show that the optimal Co₅Fe₃Cr₂ (oxy)hydroxide reaches a current density of 10 mA cm⁻² at an overpotential of 232 mV. It delivers a Co‐based mass activity of 1486 A g⁻¹ and a high turnover frequency of 0.23 s⁻¹, over two orders higher than monometallic Co (oxy)hydroxide. X‐ray absorption and photoelectron spectroscopies reveal that Cr promotes Co atoms to occupy octahedral sites and modifies the Co electronic structure. Quasi‐operando electron energy loss spectroscopy further experimentally confirm that Cr accelerates the pre‐oxidation of Co²⁺ to higher valence species, resulting in significantly higher catalytic activity. The findings here pave the way to use Co–Fe–Cr (oxy)hydroxides as an efficient OER catalyst.