Oxygen activation is a key step in the catalytic oxidation of formaldehyde (HCHO) at room temperature. In this study, we synthesized a carbon/Co₃O₄ nanocomposite (C-Co₃O₄) as a solution to the insufficient capability of pristine Co₃O₄ (P-Co₃O₄) to activate oxygen for the first time. Oxygen activation was improved via carbon preventing the agglomeration of Co₃O₄ nanoparticles, resulting in small particles (approximately 7.7 nm) and more exposed active sites (oxygen vacancies and Co³⁺). The removal efficiency of C-Co₃O₄ for 1 ppm of HCHO remained above 90%, whereas P-Co₃O₄ was rapidly deactivated. In static tests, the CO₂ selectivity of C-Co₃O₄ was close to 100%, far exceeding that of P-Co₃O₄ (42%). Various microscopic analyses indicated the formation and interaction of a composite structure between the C and Co₃O₄ interface. The carbon composite caused a disorder on the surface lattice of Co₃O₄, constructing more oxygen vacancies than P-Co₃O₄. Consequently, the surface reducibility of C-Co₃O₄ was improved, as was its ability to continuously activate oxygen and H₂O into reactive oxygen species (ROS). We speculate that accelerated production of ROS helped rapidly degrade intermediates such as dioxymethylene, formate, and carbonate into CO₂. In contrast, carbonate accumulation on P-Co₃O₄ surfaces containing less ROS may have caused P-Co₃O₄ inactivation. Compared with noble nanoparticles, this study provides a transition metal-based nanocomposite for HCHO oxidation with high efficiency, high selectivity, and low cost, which is meaningful for indoor air purification. Copyright © 2021 American Chemical Society.