The synergistic regulation of the electronic structures of transition-metal oxide-based catalysts via oxygen vacancy defects and single-atom doping is efficient to boost their oxygen evolution reaction (OER) performance, which remains challenging due to complex synthetic procedures. Herein, a facile defect-induced in situ single-atom deposition strategy is developed to anchor atomically dispersed Ru single-atom onto oxygen vacancy-rich cobalt oxides (Ru/Co₃O_4–x) based on the spontaneous redox reaction between Ru³⁺ ions and nonstoichiometric Co₃O_4–x. Accordingly, the as-prepared Ru/Co₃O_4–x electrocatalyst with the coexistence of oxygen vacancies and Ru atoms exhibits excellent performances toward OER with a low overpotential of 280 mV at 10 mA cm^–2, a small Tafel slope value of 86.9 mV dec^–1, and good long-term stability in alkaline media. Furthermore, density functional theory calculations uncover that oxygen vacancy and atomically dispersed Ru could synergistically tailor electron decentralization and d-band center of Co atoms, further optimizing the adsorption of oxygen-based intermediates (*OH, *O, and *OOH) and reducing the reaction barriers of OER. This work proposes an available strategy for constructing electrocatalysts with abundant oxygen vacancies and atomically dispersed noble metal and presents a deep understanding of synergistic electronic engineering of transition-metal-based catalysts to boost oxygen evolution.