<p>High-entropy alloys have demonstrated promising potential in water splitting catalysis owing to their unique multi-component synergistic effects, tunable electronic structures, and exceptional structural stability. In this study, self-supporting porous FeCoNi_1.4Mo_<i>x</i> (<i>x</i> = 1.0, 1.125, 1.25, 1.375, 1.5) bifunctional electrocatalysts are developed through arc melting combined with chemical dealloying, with precisely controlled compositions to tailor their microstructures. Benefiting from the porous framework and optimized component synergy, the catalysts exhibit remarkable bifunctional oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) activity in 1&#xa0;mol·L⁻¹ KOH. The FeCoNi_1.4Mo_1.125 requires merely 260.30&#xa0;mV overpotential to deliver a current density of 500&#xa0;mA·cm^‒2 for OER, while FeCoNi_1.4Mo_1.25 achieves the same current density with an overpotential of only 254&#xa0;mV for HER. These comprehensive <i>in situ</i> electron paramagnetic resonance, transmission electron microscopy, and X-ray photoelectron spectroscopy analyses consistently demonstrate that the outstanding OER performance of FeCoNi_1.4Mo_1.125 originates from its <i>in situ</i> transformation into a highly active surface composed of nanocrystalline CoOOH, NiOOH, and FeOOH. The synergistic effects among these metal oxyhydroxides create abundant active sites and facilitate charge transfer, ultimately leading to enhanced catalytic activity and stability. Notably, the assembled electrolyzer demonstrates outstanding stability, maintaining stable operation at 500&#xa0;mA·cm^‒2 for 250&#xa0;h. This performance surpasses most reported non-precious bifunctional electrocatalysts, providing new insights for designing highly stable water electrolysis catalysts and showing significant potential for industrial applications.</p> Graphical abstract <p></p>