In this study, the treatment of diclofenac (DCF), which is a pain reliever belonging to non-steroid anti-inflammatory drug group, was investigated by ozone-based oxidation processes. In experiments; pH (6-7) and 10 min reaction times were kept constant and the effects of different initial concentrations (5 mg/L, 10 mg/L and 20 mg/L), ozone doses (5 and 8 ppm), power of UV lamps (16 W and 32 W) and catalysts (0.1 g ZnO, In2O3, CuO and Fe2O3) were studied. Initially, studies were performed for the photocatalytic ozone oxidation process to determine which catalyst was more effective in DCF treatment, and study results were evaluated according to the PROMETHEE decision-making method. As a result of the evaluation based on DCF removal, energy consumption and cost parameters, ZnO was chosen as the best catalyst. In the following catalytic and photocatalytic ozone oxidation processes, studies continued with ZnO catalyst. In the second step of the study, the effect of DCF initial concentration, ozone dose and UV light intensity was determined. According to the results of the study, max degradation was obtained 100% in O-3/UV/ZnO and O-3/UV oxidation processes, at 5 mg/L DCF concentration, 5 ppm ozone dose, 16 W UV lamp and at 4- and 5-min reaction time, respectively. The highest total organic carbon (TOC) removal was obtained 76% in O-3/UV/ZnO process. O-3, O-3/UV and O-3/ZnO oxidation processes for 5 mg/L DCF were found to be better suited to pseudo-first-order kinetics rate model.