LAUSR

High‐entropy (HE) electrocatalysts are becoming a research hotspot due to their interesting “cocktail effect” and have great potential for tailored catalytic properties. However, it is still a great challenge to illustrate their inherent catalytic mechanism for the “cocktail effect”, and there is also a paucity of quantitative descriptors to characterize the specific catalytic activity and give logical design strategies for HE systems. Herein, the unexpected activation of all metal sites in HE Cu–Co–Fe–Ag–Mo (oxy)hydroxides for the oxygen evolution reaction (OER) is reported, and it is found that metal–oxygen d–p hybridization, as an effective descriptor, can indicate the intrinsic activity of each metal site. According to the quantitative hybridization, introducing an electron donor (e.g., Ag) is raised and verified to reinforce the electrocatalytic activity of the HE system. Consequently, Ag‐decorated Co–Cu–Fe–Ag–Mo (oxy)hydroxide (Ag@CoCuFeAgMoOOH) electrocatalysts are constructed by an electrochemical reconstruction method, and their OER performances are thoroughly characterized. The Ag@CoCuFeAgMoOOH is verified with a low overpotential (270 mV at 100 mA cm−2) and a small Tafel slope (35.3 mV dec−1), as well as good electrochemical stability. The favorable activity of the electron donor and underlying synergistic “cocktail effect” are demonstrated and disclosed. This work opens up a new strategy to guide the design/fabrication of advanced HE electrocatalysts.