This study investigated the effects on growth performance, histopathology, intestinal histomorphology, antioxidant activity, serum biochemical parameters, and disease resistance against A. hydrophila of carp fed with oregano essential oil (EO). Fish (13.54±0.24 g) were fed with experimental diets containing 1.0 and 3.0 mL kg−1 oregano EO for 60 days. At the end of the experiment, no significant difference was found among the growth performances of fish fed with oregano EO supplemented diets (p>0.05). However, dietary administration of 3.0 mL kg−1 oregano EO had significantly increased survival rates of common carp post-challenged with A. hydrophila (p < 0.05). In addition, goblet cells number and villi length in the intestine were increased in fish fed with oregano EO diets (p < 0.001). Also, no pathological findings were observed in hepatopancreas, gonads and intestines in fish fed with oregano EO. Fish fed with a diet containing 3.0 mL kg−1 oregano EO showed higher catalase activity (CAT) than the other experimental groups (p < 0.05). Superoxide dismutase (SOD), melondialdehyde (MDA), total protein, albumin, AST, ALT and ALP levels were similar in oregano EO supplemented groups compared to the control (p > 0.05). Number of mature oocytes in female ovaries caused a slight increase in fish fed with oregano EO. Adding oregano EO to common carp diet showed improved intestinal histomorphology and increased the survival against the A. hydrophila. Therefore, O. minutiflorum EO can be used as a health promoter for carp.