To develop highly efficient heterogeneous photocatalysts for the activation of dissolved oxygen is very interesting in the field of green degradation of organic pollutants. In the paper, oxygen atom doped g-C₃N₄ (O-g-C₃N₄) was prepared via a facile chemical oxidation of g-C₃N₄ by peroxymonosulfate. X-ray photoelectron spectroscopy analysis suggests the oxidative treatment of g-C₃N₄ by peroxymonosulfate evidently increased atomic percentage of oxygen on O-g-C₃N₄ surface to 6.9% as compared with 1.8% for g-C₃N₄. Meanwhile, the doped oxygen atom mainly existed as carbonyl and carboxyl groups. Optical characterization indicates the introduction of oxygen improved the response of O-g-C₃N₄ to visible light, and more obviously, separation of photo-generated h⁺-e⁻. 4-chloro-7-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) probe measurement indicates the formation of O₂^•− was dramatically enhanced through activation of dioxygen by photo-generated electrons in the O-g-C₃N₄ photocatalytic system. Through high performance liquid chromatography (HPLC) and Liquid chromatography–mass spectrometry (LC-MS) analysis, it was found rhodamine B (RhB) photocatalytic degradation by O-g-C₃N₄ followed step by step N-deethylation reaction pathways induced by the formed O₂^•−, rather than the non-selective decomposition of the chromophore in RhB by other radicals, such as hydroxyl radicals. This study provides a facile method to develop oxygen atom doped g-C₃N₄ photocatalyst, and also clarifies its photocatalytic activation mechanism of molecular oxygen for N-deethylation reaction of RhB.