Overlay streaming systems have recently been favored by the academic community as a viable approach for IPTV. Over the last years, a multitude of different overlay streaming approaches have been proposed. Most of them, however, have been evaluated individually. The lack of a common simulation framework makes it difficult to compare the properties of the different systems with each other. To bridge this gap, we introduce OSSim, a general-purpose simulation framework that allows the instantiation of different overlay streaming protocols. For this purpose, it provides a generic and modular structure, and several membership management and overlay streaming protocols as well. Our simulation results indicate that the framework is accurate and flexible to simulate different overlay streaming systems.