Photocatalytic O₂ activation to generate reactive oxygen species is crucially important for purifying organic pollutants, yet remains a challenge due to poor adsorption of O₂ and low efficiency of electron transfer. Herein, we demonstrate that ultrafine MoO_x clusters anchored on graphitic carbon nitride (g-C₃N₄) with dual nitrogen/oxygen defects promote the photocatalytic activation of O₂ to generate ·O₂⁻ for the degradation of tetracycline hydrochloride (TCH). A range of characterization techniques and density functional theory (DFT) calculations reveal that the introduction of the nitrogen/oxygen dual defects and MoO_x clusters enhances the O₂ adsorption energy from −2.77 to −2.94 eV. We find that MoO_x clusters with oxygen vacancies (Ov) and surface Ov-mediated Mo^δ+ (3 ≥ δ ≥ 2) possess unpaired localized electrons, which act as electron capture centers to transfer electrons to the MoO_x clusters. These electrons can then transfer to the surface adsorbed O₂, thus promoting the photocatalytic conversion of O₂ to ·O₂⁻ and, simultaneously, realizing the efficient separation of photogenerated electron–hole pairs. Our fully-optimized MoO_x/g-C₃N₄ catalyst with dual nitrogen/oxygen defects manifests outstanding photoactivities, achieving 79% degradation efficiency toward TCH within 120 min under visible light irradiation, representing nearly 7 times higher activity than pristine g-C₃N₄. Finally, based on the results of liquid chromatograph mass spectrometry and DFT calculations, the possible photocatalytic degradation pathways of TCH were proposed.