Operation policies of reservoirs may be generated by simulation or optimization methods. Water resources management students and/or planner often have difficulties employing modern heuristic optimization methods such as Genetic Algorithm (GA) and Particle Swarm Optimization (PSO) to produce optimum operation policies. ResOS is a web application which freely provides simulation and optimization tools (e.g. GA) to produce operation policy for reservoirs. ResOS is an online app and can be used on all internet connected devices such as computers or cell phones. ResOS requires no minimum hardware configuration to run because all calculation modules run on the server side and user device only displays the results. ResOS provides a free and user-friendly environment for students to have a full access to GA, PSO, etc, so that they can investigate behaviors of such algorithms in solving reservoir operation optimization problems, and also actually use it to generate operation policies for case studies.