Nickel-based layered hydroxides (LHs) are a family of efficient electrocatalysts for the alkaline oxygen evolution reaction (OER). Nevertheless, fundamental aspects such as the influence of the crystalline structure and the role of lattice distortion of the catalytic sites remain poorly understood and typically muddled. Herein, we carried out a comprehensive investigation on ɑ-LH, β-LH and LDH phases, analysing the role exerted by Ni-vacancies by means of structural, spectroscopical, in-silico and electrochemical studies. Indeed, density functional theory (DFT) calculations, in agreement with X-ray absorption spectroscopy (XAS), confirm that the presence of Ni-vacancies produces acute distortions of the electroactive Ni sites (shortening in the Ni-O distances and changes in the O-Ni-O angles), triggering the appearance of Ni localised electronic states on the Fermi level, reducing of Egap, and therefore increasing the reactivity of the electroactive sites. Furthermore, post-mortem Raman and XAS measurements unveil the transformation of ɑ-LH phase into a highly reactive oxyhydroxide-like structure stable under ambient conditions. Hence, this work pinpoints the critical role of cationic vacancies on the structural and electronic properties of the LH structures, which controls their inherent reactivity towards OER catalysis. We envision Ni-based ɑ-LH as a perfect platform for trivalent cations hosting, closing the gap toward the next generation of benchmark efficient earth-abundant electrocatalysts.