The motivation behind this article is to give point-by-point data about the benefi- cial applications of oregano feed supplement in fish diets as immunomodulators, antioxidant, antiviral, antifungal and antiparasitic. Use of this plant as feed addi- tive plays an important role in the fish diet when compared to other synthetic feed additives. Oregano is rich in phytochemical compounds including carvacrol and thymol in addition to other phenolic compounds with antioxidant and immune- enhancing activities. Origanum vulgare extract improved the immunological responses and enhanced non-specific immunity. Also, non-specific immunity and the lysosomal activity were significantly increased in rainbow trout fed diet enriched with 3.0 mL essential oil of Origanum onites Lkg
1 diet for 60 days. Furthermore, non-specific immune stimulant, antioxidant and nitric oxide activi- ties were improved due to O. vulgare oil supplementation. In some recent studies, Origanum heracleoticum L essential oil as a growth enhancer increased the antiox- idant status. In rainbow trout, the hepatic levels of antioxidant enzymes and the total antioxidant capacity increased by feeding diet enriched with 6 and 10 g kg
1 diet of O. vulgare extract. Therefore, the addition of oregano and/or derivatives as a dietary supplement in fish diet may promote growth and enhance the immunity and health of fish and this will be useful for nutritionists, physiologists and veteri- narians.