Ozone has been widely used for water treatment all over the world. This study investigated the effects of pre-ozonation on N-nitrosodimethylamine (NDMA) formation from four drinking-water sources during subsequent chloramination. Moreover, relationships between NDMA and water-quality parameters (dissolved organic carbon (DOC), dissolved organic nitrogen (DON), hydrophobic fractions (HFs), positive-charged fractions (PCFs), ultraviolet absorbance at 254 nm wavelength (UV254), ammonia) were determined. The results indicated that NDMA formed directly by ozonation, but the concentrations (5.7–37.8 ng/L) were much lower than during chloramination (25.3–193 ng/L). Pre-ozonation slightly augmented NDMA formation from reservoirs, while it facilitated NDMA precursor destruction in the lake and river. The river was rich in organic matter, its DOC concentration was up to 4.97 mg/L, while the DON/DOC ratio (4.68%) was low; the lake possessed the most organic nitrogen and highest DON/DOC ratio (10.76%). HFs in reservoir #2 were low (1.27 mg/L), and the HFs/DOC ratio was especially high (73.4%). PCFs constituted 48.5%–72.1% of DOC. UV254 values were in the range 0.081–0.175 cm−1. The ammonia concentration was high in the lake water (0.29 mg/L), while that in reservoir #2 was only 0.08 mg/L. There was a strong correlation between NDMA formation and the removal of ammonia, DON and PCFs.