Effects of Origanum vulgare oil on oxidant antioxidant system in pentachlorophenol-given rat were investigated. Total thirty-two Wistar Albino race male rats were used. Rats were divided into four even groups. Group 1 was assigned control. Groups 2, 3, and 4 were given Origanum vulgare oil at the dose of 1.5 ml/kg.bw/day, pentachlorophenol at the dose of 60 mg/kg.bw/day and Origanum vulgare oil at the dose of 1.5 ml/kg.bw/day plus pentachlorophenol at the dose of 60 mg/kg.bw/day into the stomach directly, respectively, for 21 days. At the end of the 21th day, tissues (liver, kidney, brain) and blood samples of all groups were taken. Malondialdehyde (MDA) and nitric oxide (NO) levels and superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GSH-Px) activities were measured in the all samples. As a result, while pentachlorophenol exposure caused oxidative damage to erythrocytes and some tissue of rats at different rates, Origanum vulgare oil alleviated the severity of pentachlorophenol-induced oxidative stress. Origanum vulgare oil alone also has not any negative effects. In conclusion, Origanum vulgare oil can be applied as a therapeutic agent or food additive for prophylaxis against exposure to pentachlorophenol.