Abstract : OSA (Open Simulation Architecture) is a collaborative platform for component-based discrete-event simulation. It has been created to support both M&S studies and research on M&S techniques and methodology. The OSA project started from the observation that despite no single simulation software seems to be perfect, most of the elements required to make a perfect simulator already exist as part of existing simulators. Hence, the particular area of research that motivated the OSA project is to investigate practical means of reusing and combining any valuable piece of M&S software at large, including models, simulation engines and algorithms, and supporting tools for the M&S methodology. To achieve this goal, the OSA project investigates in advanced software engineering techniques such as component-based framework, layered patterns and aspect-oriented programming. In cases studies, the OSA project is among others involved in a large-scale simulation, and a distributed simulation over the RESTful protocol.