This study provided a preliminary framework for the effects of Ropadiar (Origanum vulgare L.) on Nile tilapia (Oreochromis niloticus) on some immune parameters over a 60-day feeding trial and after bacterial challenge with Aeromonas hydrophila. A total of 60 O. niloticus 14.5-15.3 g were divided into two groups (10 fish/ aquarium/3 replicate). The first group was fed a basal diet, which served as control, the second group was fed 2% Oreganum oil added to the diet for 60 days, then they were inoculated with Aeromonas hydrophila. Hematological parameters and cytokines levels, phagocytic index and nitric oxide level (NO) as well as serum lysozyme activity were determined. Histopathology for the spleen was done. Results revealed significant improvement in hemoglobin (HB), red blood cells (RBCs) and white blood cells (WBCs) count, lysozyme, phagocytic activity, nitric oxide levels and cytokines up-regulation in 2% oreganum oil supplemented group than the control. Histopathology for the spleen was significantly improved in 2% of the oreganum oil-supplemented group compared to the control. The incorporation of Ropadiar 2% in the diet of Nile tilapia is recommended to improve hematological, histopathological and immune parameters in Nile tilapia.