Abstract: Even in their bulk forms, complex alloys like high-entropy alloys (HEAs) exhibit favorable activity and stability as electrocatalysts for the oxygen evolution reaction (OER). However, the underlying reasons are not yet fully understood. In a family of Mo-doped CrFeCoNi-based HEAs, we have identified three crucial factors that govern their performance: (i) homogeneous solid solution phase of HEAs helps to maintain high-valence states of metals; (ii) surface reconstruction results in a hybrid material comprising amorphous domains and percolated crystalline structures; (iii) diversity of active intermediate species (M-O, M-OOH, and, notably, the abundance of superoxide -OO), which display stronger adsorption capacity on the reconstructed surface. These results are revealing due to their resemblance to findings in other families of electrocatalysts for OER, as well as their unique features specific to HEAs. In line with these factors, a CrFeCoNiMo_0.2 bulk integrated electrode displays a low overpotential of 215 mV, rapid kinetics, and long-term stability of over 90 d. Bulk HEAs hold great potential for industrial applications.