One-day-old Cherry valley meat-strain ducks were used to investigate the effect of supplemental dried oregano powder (DOP) in feed on the productivity, antioxidant enzyme activity, and breast meat quality. One hundred sixty five ducks were assigned to 5 dietary treatments for 42 days. The dietary treatment groups were control group (CON; no antibiotic, no DOP), antibiotic group (ANT; CON+0.1% Patrol), 0.1% DOP (CON+0.1% DOP), 0.5% DOP (CON+0.5% DOP), and 1.0% DOP (CON+1.0% DOP). Upon feeding, 1,1-diphenyl-2-picryl-hydrazyl (DPPH) radical scavenging activity of oregano extracts was higher than that of tocopherol, although it was less than that of ascorbic acid. As a result of in vivo study, DOP in the diet showed no effects on final body weight, feed intake, or feed conversion ratio. However, dietary 0.5% and 1% DOP supplementation caused a significant increase in the serum enzyme activity of superoxide dismutase (SOD) compared with CON and ANT, while glutathione peroxidase (GPx) in tissue was increased as compared to ANT (p<0.05). Cooking loss from ducks fed with DOP decreased compared with the control ducks. Thiobarbituric acid reactive substance (TBARS) values of duck breast meat at 5 d post slaughter was found to be significantly reduced in ducks whose diets were supplemented with 0.5% and 1% DOP (p<0.05). These results suggest that diets containing 0.5% and 1% DOP may beneficially affect antioxidant enzyme activity of GPx and SOD, improve meat cooking loss, and reduce TBARS values in breast meat at 5 d of storage in ducks.