Introduction: Unilateral ureteral obstruction (UUO) is one of the causes of kidney injury. OLEUROPEIN is known as one of the strongest natural antioxidants. The aim of this study was to determine the effect of complete left unilateral ureteral obstruction on the antioxidant system changes in the contralateral kidney in male rats with an emphasis on the protective role of OLEUROPEIN Materials and Methods: In this experimental study, 18 male Wistar rats weighing 200 to 250 g were used. The animals were divided into 3 groups of 6 which included control, UUO, and UUO + OLEUROPEIN. The left ureter was ligated for 3 days. In the treatment group, the animals received OLEUROPEIN (200 mg/kg) daily for 3-days from one hour after ureteral obstruction. At the end of the experiment, the right kidney was removed and weighed. Then, the malondialdehyde (MDA) concentration, glutathione peroxidase (GPx), and superoxide dismutase (SOD) enzymes activity levels were measured in the right kidney. Results: UUO significantly increased right kidney weight and MDA concentration, and decreased the SOD and GPx enzymes activity level in the right kidney of UUO animals compared to the control (p<0. 05). OLEUROPEIN was able to reduce right kidney weight and MDA concentration in the UUO+OLEUROPEIN group compared to the UUO group (p<0. 05), and significantly increased the SOD and GPx enzymes activity level (p<0. 05). Conclusion: UUO induces oxidative stress in the contralateral kidney, and OLEUROPEIN administration can have a protective effect on the contralateral kidney by restoring the antioxidant system.