The lack of a single system image Operating System (OS) for clusters restricts their use for parallel processing. We propose an approach for building an efficient single system image cluster operating system. The proposed system implements distributed services performing global and dynamic resource management to offer high performance, high availability and ease of use and programming. The programming API of the OS running on each cluster node is kept unmodified but the high level OS services can take benefit of all cluster resources. Our approach has been validated by a prototype based on Linux. Our prototype comprises of a limited kernel patch and a set of modules extending the kernel to implement the cluster distributed services. Existing applications running on symmetric multiprocessors (SMP) on top of Linux can be executed on top of our cluster OS without modification.