This work was carried out to evaluate the efficacy of oregano powder 10% used as a prophylactic treatment in broiler chickens experimentally infected with avian pathogenic E. coli O27 (APEC O27) strain -local isolate-. Hematological and serum biochemical parameters, as well as histopathology of the vital organs, were investigated. A total of 200 chickens -aged 19 days old- were randomly allocated into 4 equal groups; Normal control (G1), Infected (G2), Oregano control (G3), and Oregano-Infected (G4). Chickens of (G3) and (G4) were supplemented with oregano powder that contains 10% oregano essential oil in the diet (250 g /ton diet) from the 1st day old till the end of the experiment. At the age of the 19th, 20th, and 21st day, chickens of (G2), and (G4) were infected with 0.2 ml of APEC O27 (1×107 CFU/chick) via the intratracheal route. On the 3rd, 6th, 10th, and 21st day post-infection, blood and tissue samples from all groups were collected for laboratory examinations of the hemogram, serum biochemistry, and histopathology of the vital organs. Chickens of (G2) showed gasping, dullness, loss of appetite, and ruffled feathers. Birds of (G3) showed no symptoms, while those of G4 showed mild symptoms. The mortality rates in chickens of (G2), (G3), and (G4) were recorded 30%, 0%, and 4%, respectively. Birds of (G2) revealed macrocytic hypochromic anemia, leukocytosis accompanied by heterophilia, and lymphopenia. The activities of AST and ALT enzymes and the values of uric acid and creatinine were increased while those of protein profile were decreased. In oregano supplemented groups, no significant alterations were detected. Birds of (G2) and (G4) revealed histopathological alterations in the lungs, trachea, liver, spleen, and kidneys. Finally, APEC O27 caused changes in blood picture, liver and kidney functions as well as histopathology of the internal organs. Oregano powder (at a dose of 10% in diet) could be used as prophylactic treatment for APEC O27 in chickens from the 1st day of age.