Project XOS or eXperimental Operating System is a platform to develop a toy operating system. It is an instructional tool for students to learn and implement OS data structures and functionalities on a simulated machine called XSM (eXperimental String Machine).

The OS is programmed using a custom language known as SPL (System Programmer's Language) and application programs, which run on the OS, are programmed using APL (Application Programmer's Language).