Photocatalytic oxidation technology for the anoxic removal of organic pollutants that exist under some oxygen-free conditions is attractive but challenging. In this work, oxygen doped graphitic carbon nitride (g-C₃N₄) with outstanding visible light activity under anoxic conditions is synthesized via oxygen-plasma treatment for the first time. Oxygen doping does not influence the structure of g-C₃N₄ but changes its morphology, enhances the S_BET, decreases the band gap energy and increases the separation efficiency of photogenerated electrons and holes, which increase anoxic photocatalytic RhB degradation constants by approximately 6 times. After plasma treatment, doped oxygen not only increases the adsorption ability of g-C₃N₄ but also captures photogenerated electrons to reserve photogenerated holes for RhB degradation under anoxic conditions. This study provides a new insight into the design and fabrication of oxygen-free photocatalysts.