We apply Discrete Event Simulation (DES) to a system of two strongly interconnected departments, an outpatient oncology clinic for chemotherapy delivery, and the pharmacy unit that prepares the chemotherapy drugs. The model is developed in close collaboration with the French hospital Henri Mondor, and is validated using real data. The objective is to identify sources of patient waiting times in the outpatient oncology clinic and to identify relevant corrective actions. We show that the coordination between the two departments is the key barrier to higher performance. Solutions are proposed based on increased information sharing and obtaining advanced information on patient status, to allow advanced drug preparation. A two-phase project for improvement is proposed. This paper contributes to the literature on multi-department simulation, which is still rare in healthcare OR compared to one-department studies.