This paper presents an outcome of a workflow research project that will allow hospital organizations to master their costs and to improve their quality of service. Indeed, to reach this objective, the hospital business processes have to be modelled. This work is illustrated here through the analysis of operating room processes modelled in terms of Petri nets. After soundness verification of the procedures underlying these business processes, we show how we can optimise the performances of a system by performing process reengineering. Due to the context, this optimisation is achieved by introducing a balance between specialization and generalization and between centralization and decentralization of resource classes (equipment and staff) while preserving the soundness property.