The sluggish kinetics of the oxygen evolution reaction (OER) is the bottleneck for the practical exploitation of water splitting. Here, the potential of a core-shell structure of hydrous NiMoO microrods conformally covered by CoO nanoparticles via atomic layer depositions is demonstrated. In situ Raman and synchrotron-based photoemission spectroscopy analysis confirms the leaching out of Mo facilitates the catalyst reconstruction, and it is one of the centers of active sites responsible for higher catalytic activity. Post OER characterization indicates that the leaching of Mo from the crystal structure, induces the surface of the catalyst to become porous and rougher, hence facilitating the penetration of the electrolyte. The presence of CoO improves the onset potential of the hydrated catalyst due to its higher conductivity, confirmed by the shift in the Fermi level of the heterostructure. In particular NiMoO@CoO shows a record low overpotential of 120 mV at a current density of 10 mA cm, sustaining a remarkable performance operating at a constant current density of 10, 50, and 100 mA cm with negligible decay. Presented outcomes can significantly contribute to the practical use of the water-splitting process, by offering a clear and in-depth understanding of the preparation of a robust and efficient catalyst for water-splitting.