TunableegOrbital Occupancy in Heusler Compounds for Oxygen Evolution Reaction**

Heusler compounds have potential in electrocatalysis because of their mechanical robustness, metallic conductivity, and wide tunability in the electronic structure and element compositions. This study reports the first application of Co2YZ‐type Heusler compounds as electrocatalysts for the oxygen evolution reaction (OER). A range of Co2YZcrystals was synthesized through the arc‐melting method and theegorbital filling of Co was precisely regulated by varyingYandZsites of the compound. A correlation between theegorbital filling of reactive Co sites and OER activity was found for Co2MnZcompounds (Z=Ti, Al, V, and Ga), whereby higher catalytic current was achieved foregorbital filling approaching unity. A similar trend ofegorbital filling on the reactivity of cobalt sites was also observed for other Heusler compounds (Co2VZ,Z=Sn and Ga). This work demonstrates proof of concept in the application of Heusler compounds as a new class of OER electrocatalysts, and the influence of the manipulation of the spin orbitals on their catalytic performance.