Photocatalytic ozonation of wastewater pollutants by sunlight is a highly attractive technology close to real application. Understanding this process on the atomic scale and under realistic working conditions is challenging but vital for the rational design of catalysts and photocatalytic decontamination systems. Here we study two highly active C3N4 photocatalysts (bulk C3N4 and a nanosheet-structured C3N4) under simultaneous visible-light irradiation and O-3 bubbling in water by in situ EPR spectroscopy coupled with an online spin trapping technique. The photoexcitation of electrons to the conduction band (CB-e(-)), their further trapping by dissolved O-2 and O-3, and the evolution of reactive oxygen species (ROS) have been semiquantitatively visualized. A dual role of O-3 in boosting the CB-e(-) to (OH)-O-center dot conversion is confirmed: (i) an inlet 2.1 mol % O-3/O-2 gas mixture can trap about 2-3 times more CB-e(-) upon aqueous C3N4 suspension than pure O-2 and further produce (OH)-O-center dot by a robust O-center dot(3)--mediated one-electron-reduction pathway (O-3 -> O-center dot(3)- -> HO3 center dot -> (OH)-O-center dot); (ii) O-3 can readily take CB-e(-) back from O-center dot(3) to form O-center dot(3)-, thus blocking the inefficient H2O2-mediated three-electron-reduction route (O-2 -> O-center dot(2)- -> HO2 center dot H2O2 -> (OH)-O-center dot) but further strengthening the O-center dot(3)--mediated pathway. In the presence of 2.1 mol % O-3/O-2, the (OH)-O-center dot yield increases by 17 and 5 times, and consequently, the mineralization rate constant of oxalic acid increases by 84 and 41 times over bulk C3N4 and NS C3N4, respectively. This work presents an attractive opportunity to boost the yield of ROS species ((OH)-O-center dot) for water purification by visible-light-driven photocatalysis and provides a powerful tool to monitor complex photocatalytic reactions under practical conditions.