Ozone (O3) is widely used in drinking water disinfection and wastewater treatment. However, when applied to bromide-containing water, ozone induces the formation of bromate, which is carcinogenic. Our previous study found that graphene oxide (GO) can enhance the degradation efficiency of micropollutants during ozonation. However, in this study, GO was found to promote bromate formation during ozonation of bromide-containing waters, with bromate yields from the O3/GO process more than twice those obtained using ozone alone. The promoted bromate formation was attributed to increased hydroxyl radical production, as confirmed by the significant reduction (almost 75%) in bromate yield after adding t-butanol (TBA). Cerium oxide (less than 5 mg/L) supported on reduced GO (xCeO2/RGO) significantly inhibited bromate formation during ozonation compared with reduced GO alone, and the optimal Ce atomic percentage (x) was determined to be 0.36%, achieving an inhibition rate of approximately 73%. Fourier transform infrared (FT-IR) spectra indicated the transformation of GO into RGO after hydrothermal treatment, and transmission electron microscope (TEM) results showed that CeO2 nanoparticles were well dispersed on the RGO surface. The X-ray photoelectron spectroscopy (XPS) spectra results demonstrated that the Ce3+/Ce4+ ratio in xCeO2/ RGO was almost 3–4 times higher than that in pure CeO2, which might be attributed to the charge transfer effect from GO to CeO2. Furthermore, Ce3+ on the xCeO2/RGO surface could quench Br· and BrO· to further inhibit bromate formation. Meanwhile, 0.36CeO2/RGO could also enhance the degradation efficiency of N,N-diethyl-m-toluamide (DEET) in synthetic and reclaimed water during ozonation.