performance can be limited due to the slow oxidation kinetics and consequent low degree formation of MOOH intermediates. [16,17] As such, the matter of 3D transition metals in OER activity may come down to their facile oxidation, referring to the necessity of novel structural design to tune the oxidation kinetics. Recently, heterogeneous catalysts-supported metals have shown notable OER activities resulting from the similarity of metal crystallite to support materials and catalytic synergy of support species support to the 3D metal. [18] The synergistic effect refers to modification of electronic structures of both support materials and anchored metals, which leads to the acceleration of oxidation kinetics of the anchored metal. [19] This infers that the introduction of conductive support for transition metals could help to readily transport electrons from the transition metal particles to support materials. Pyrochlore oxides (A 2 B 2 O 7 ) have a characteristic that electrons generated during oxygen evolution (4OH − → O 2 + 2H 2 O + 4e − ) are rapidly transferred from surface to inner layers with low-resistance pathway by oxidizing either A or B-site metal cation of pyrochlore oxide. [20][21][22][23][24][25] As such, the metallic conduction pyrochlore oxide can be a suitable support for transition metals, which provide a rationale for transitionmetal anchored pyrochlore oxide for OER. In the integrated design of transition metals and pyrochlore oxides, the pyrochlore oxide support effectively transfer the electrons generated during the oxidation of transition metal to pyrochlore oxide inner layers, reducing the barrier energy for the formation of MOOH intermediates.For the synthesis of metal-supported heterogeneous structure, in situ exsolution method is widely adopted in that metal nanoparticles are uniformly grown on support materials. [26][27][28][29][30] Recently, CoFe nanoalloys decorated on La 2 O 3 has been reported where the CoFe alloy particle is formed by the reduction of Co and Fe cations from LaCo 0.8 Fe 0.2 O 3−δ perovskite oxide. [31] The CoFe/La 2 O 3 catalyst showed notable OER potential of 1.52 V versus RHE at the current density of 10 mA cm −2 in alkaline medium, showing the transformation of CoFe nanoalloys to (Co/Fe)O(OH) intermediation. However, the OER active sites of the CoFe/La 2 O 3 only come from the CoFe nanoalloys, for the perovskite oxide (ABO 3 ) support are transformed to OER inactive metal oxide (i.e., La 2 O 3 ). The reduction of all the Co It is crucial for leaping forward renewable energy technology to develop highly active oxygen evolution reaction (OER) catalysts with fast OER kinetics, and the novel design of high-performance catalysts may come down to unveiling the origin of high catalytic behavior. Herein, a new class of heterogeneous OER electrocatalyst (metallic Co nanoparticles anchored on yttrium ruthenate pyrochlore oxide) is provided for securing fast OER kinetics. In situ X-ray absorption spectroscopy (in situ XAS) reveals that fast OER kinetics can be achieved by t...