Objectives: Identify the in vitro synergistic effect of the Origanum vulgare, “oregano” essential oil with ciprofloxacino, against ampicillin-resistant E. coli, to determine its Minimum Inhibitory Concentration (MIC) and to determine the susceptibility of 50% and 75% essential oil concentrations in synergy with ciprofloxacin against ampicillin-resistant E. coli. Material and Method: The essential oil extraction from Origanum vulgare leaves was carried out using the steam distillation method. Subsequently many separations were done at 5%, 10%, 15%, 20%, 50% and 75% concentrations. The E. coli ampicillin-resistant strains were cultivated and seeded in Petri dishes containing Mueller Hinton Agar. The susceptibility test was then performed using the Kirby and Bauer technique by means of a Whatman No. 4 sterile filter paper discs, which were immersed into each of the essential oil concent...