The sluggish kinetics of the oxygen evolution reaction (OER) severely limits the efficiency of electrochemical water splitting for sustainable hydrogen production. Developing cost-effective and efficient OER electrocatalysts based on earth-abundant elements is thus highly desirable. Herein, we report a nanoporous (CoNiFe)OOH electrocatalyst decorated with Zn(OH)₄²⁻ anions, synthesized via electrochemical surface reconstruction of ZnO-decorated CoNiFe medium-entropy alloys (MEAs). The reconstructed (CoNiFe)OOH adsorbed with Zn(OH)₄²⁻ anions serves as the real active phase, featuring abundant catalytic sites and enhanced OH⁻ accessibility. Adsorbed Zn(OH)₄²⁻ anions promote OH⁻ transfer and facilitate electron redistribution at the active sites, particularly enhancing Co site activity, as revealed by density functional theory (DFT) calculations. As a result, the optimized CoNiFeZn@NF-EO electrode exhibits outstanding OER performance, achieving a low overpotential of 264 mV at 10 mA·cm⁻², a Tafel slope of 46.6 mV·dec⁻¹, and remarkable long-term stability in alkaline electrolyte. This work provides new insights into the synergistic effect between surface reconstruction and Zn-based species, offering a promising strategy for designing high-performance OER electrocatalysts.