Abstract This paper introduces M_OBS_SIM, a network simulator conceived to evaluate the performance of networks where the Optical Burst Switching (OBS) paradigm is implemented. Its accurate design allows to investigate the behavior of optical edge and core routers, as well as to assess the performance of the whole optical network; its modular construction guarantees a high degree of flexibility and a facilitated introduction of further developments. Its functional blocks, properly configured, allow to explore the effects that the most salient features of an OBS network – input traffic statistics, assembly algorithms, resource reservation policies, routing strategies – have on user performance. New and valuable results for different classes of applications with diverse service requirements are provided and discussed.