The development of oxygen evolution reaction (OER) electrocatalysts with exceptional activity and robust corrosion resistance is pivotal for advancing water and seawater electrolysis toward sustainable hydrogen production. Herein, copper-doped amorphous high-entropy hydroxide (Cu-NiCoFeCrOH) is constructed via one-step electrodeposition on nickel foam, specifically designed to enhance water/seawater oxidation performance. Comprehensive electrochemical analyses further demonstrate that the Cu doping significantly promotes electrochemical reconstruction by accelerating Cr leaching and re-adsorption, which boosts active oxyhydroxide formation, optimizes electronic structure, and facilitates OH- and *OH adsorption. These effects collectively facilitate the activation of the lattice oxygen oxidation mechanism (LOM), as evidenced by mechanistic studies. Furthermore, the catalyst exhibits excellent corrosion resistance due to the chromate re-adsorption layer, unraveled by the obvious negative shifts of point of zero charge (PZC) and open-circuit potential (OCP) with increasing KOH concentration. Consequently, the Cu-NiCoFeCrOH catalyst delivers superior OER performance with low overpotentials of 209.2 and 221.2 mV at 10 mA cm-2 in alkaline freshwater and simulated seawater, respectively, along with excellent stability (200 mA cm-2@100 h). This work provides valuable insights for the rational design and fabrication of highly active and corrosion-resistant electrocatalysts tailored for water and seawater oxidation applications.