Ozone is a powerful oxidant widely used in water treatment for the degradation of organic pollutants and removal of colour, odor, and pathogens. In aqueous solution, ozone decomposes to generate hydroxyl radicals through chain reactions that are accelerated by the addition of hydrogen peroxide in the peroxone process. Yet the precise initiation mechanisms of these chains and the efficiency of hydroxyl radical production have remained controversial, with prior models proposing adduct formation as the rate-limiting step and yielding only approximately 50% hydroxyl radicals in peroxone process. Here we show that the hydroxyl radical yield in the peroxone reaction is approximately 67%, substantially higher than previously reported. Through complete-capture scavenger assays, competition experiments, and high-precision quantum-chemical calculations informed by Marcus electron-transfer theory, we establish that ozone reacts with hydroxide exclusively by oxygen-atom transfer, while its reaction with the hydroperoxide anion proceeds through parallel electron transfer (approximately 50%) and oxygen-atom transfer (approximately 50%) pathways. Spin-orbit coupling enables spin-forbidden release of triplet oxygen in the atom-transfer channel. We also determine the pK_a of the hydroxyl radical precursor hydrotrioxide as approximately 6.15 and quantify the long-disputed hydroxyl radical–ozone reaction rate constant as 1.1 × 10⁸ M⁻¹ s⁻¹. These results revise classical ozonation and peroxone mechanisms and provide a mechanistic foundation for optimizing ozone-based advanced oxidation technologies for water purification.