High-entropy oxides (HEOs) have emerged as promising electrocatalysts for the oxygen evolution reaction (OER) owing to their unique multi-element synergy, yet precisely regulating reaction pathways to balance activity and stability is highly challenging. Herein, a spinel-type FeCoNiCrRu HEO is synthesized via an ultrafast thermal shock method, enabling synergistic dual-pathway catalysis involving the adsorbate evolution mechanism (AEM) and lattice oxygen mechanism (LOM). In situ differential electrochemical mass spectrometry, in situ Raman spectroscopy, and theoretical calculations reveal that Cr and Ru jointly enhance OER kinetics under the AEM pathway at the Co sites, while Ru induces a localized LOM pathway occurring at the Fe–Cr bridging oxygen sites, featuring optimized intermediate adsorption, upshifted O 2p band, and enhanced metal–oxygen covalency. The selective lattice oxygen activation and high-entropy nature ensure exceptional structural stability. Consequently, the FeCoNiCrRu HEO achieves a low overpotential of 200 mV at 10 mA cm−2 and extraordinary durability over 500 h at 500 mA cm−2 in alkaline media. The assembled anion exchange membrane water electrolyzer delivers 1.0 A cm−2 at 2.03 V and operates robustly for 400 h. This work successfully unifies the merits of the AEM and LOM, providing a strategic design to overcome the fundamental activity–stability trade-off in OER catalysis.