The present article is the result of a collaboration between scientists of two CNR Institutes (ISM and IMM), of the Luleå University of Technology (Sweden) and of the Ca’ Foscari University of Venice. Our complementary competences in the field of micro- and nano-materials were combined to synthesize and characterize a structure formed by nickel molybdate (NiMoO₄) micro-rods covered with a thin layer of Co₃O₄. This core-shell structure turns out to be a highly efficient electrocatalyst for oxygen evolution reaction (OER). The origin of this behavior was investigated by multiple experimental techniques, including high-resolution photoemission spectroscopy at the VUV-Photoemission beamline (Elettra synchrotron, Trieste), and ascribed to the formation of an irreversible reconstruction of the catalyst. In-situ Raman measurements show the presence of a newly evolved NiOOH Raman peak. In parallel, the core level and valence band analyses reveal shifts of all spectra features towards lower binding energies, which indicate the development of a more metallic character of the catalysis after the OER test.