The Optical System Simulator (OSS) provides a comprehensive platform for the analysis and design of isolated optical devices and entire optical systems, with special focus on semiconductor lasers and system exhibiting fast dynamics. The OSS is able to simulate "1-D" (or fundamental mode) optical devices, such as ideal optical sources, narrow stripe semiconductor lasers, wave-guides, passive cavities, mirrors, beam splitters, etc. These individual elements can be combined to form into complex optical systems. Once the optical system is defined, the user can interact with the simulation interactively using a set of Graphical User Interfaces (GUIs). These provide a visual representation of user-defined sensor outputs, and allow on-the-fly modification of system parameters, in a manner similar to that of an experimental setup. Additionally, the user can use the simulator in a background mode, with no user interfaces required, allowing very long simulations that can run unattended for hours or days. The users may reattach the GUIs at any time to monitor the simulation progress or to modify system parameters. The OSS provides a unique software simulation environment for teaching, research and design of broadband integrated optical and opto-electronic systems, where several Terahertz of optical bandwidth needs to be resolved, from femtoseconds to milliseconds time scales.