Previous in vitro studies have shown that oxidized low-density lipoprotein (ox-LDL) plays a role in the pathogenesis of osteoarthritis (OA). Paraoxonase-1 (PON1) protects both low-density lipoproteins (LDLs) and high-density lipoprotein (HDLs) against oxidative damage from circulating cells. In addition, PON1 is inactivated by ox-LDL and preserved by antioxidants. However, the relationship between serum ox-LDL, oxidative stress, and PON1 in knee OA remains unclear. Therefore, we investigated ox-LDL association with oxidative stress and PON1 in knee OA, and evaluated their relationships using radiological and clinical parameters. This study included 203 patients and 194 controls. The severity of OA was classified based on the Kellgren-Lawrence scoring system. In addition, each patient was clinically evaluated using the Western Ontario and McMaster University Osteoarthritis Index (WOMAC) score. Plasma concentrations of ox-LDL, oxidative stress markers, and PON1 were measured. Serum ox-LDL and oxidant parameters were significantly higher in patients compared to controls (p < 0.001 for all), whereas PON1 was significantly lower (p < 0.001). ox-LDL was inversely correlated with PON1, whereas it was positively correlated with radiographic severity, WOMAC score, and oxidant parameters. We found an association between the levels of various serum markers of oxidative injury, especially ox-LDL, and increasing severity of knee OA, as well as indirect evidence for their regulation by PON1. oxLDL seems to play a critical role in OA, both in the beginning, and during progression of, the disease. Therefore, serum oxLDL levels may be a helpful biomarker to evaluate the severity of knee OA.