This study aimed to evaluate the efficacy of oregano oil 20% for the treatment of broiler chickens experimentally infected with avian pathogenic Escherichia coli O27 (APECO27) -local isolate. Hemogram, biochemical constituents, and histopathology of the internal organs were assessed. Two hundred chicks -19-days-old- were randomly assigned to four equal groups; G1 (normal control), G2 (Infected), G3 (oregano treated), and G4 (antibiotic treated). At the 19th, 20th, and 21st day of age, G2, G3, and G4 were intratracheally infected with 0.2ml of APECO27 (1×107 CFU/chick). On the 22nd, 23rd, and 24th day of age, G3 & G4 were treated with oregano 20% (2ml/5liter) and difloxacin (10mg/kg b.wt.) in drinking water, respectively. On the 3rd, 6th, 10th, and 21st day post-infection, blood and tissue samples were collected for laboratory investigations of the hemogram, serum biochemistry, and histopathology of the internal organs. Infected non-treated G2 chicks showed dullness, ruffled feathers, gasping, and loss of appetite, while infected-treated G3 & G4 birds showed mild symptoms. The mortality rate was 30%, 8%, and 4% in G2, G4, and G3, respectively. Chicks of G2 & G4 developed macrocytic hypochromic anemia, leukocytosis, heterophilia, and lymphopenia. Values of ALT, AST, creatinine, and uric acid were increased while protein profile was decreased. After oregano treatment, alterations were returned to normal levels. Histopathologically, chicks of G2 & G4 showed alterations in the internal organs. Finally, APECO27 caused alterations in hematology and serum biochemistry, and histopathology of the internal organs. Oregano could be used as antibiotic for the treatment of APECO27.