Classical system development techniques often cannot cope with the ever increasing size and complexity of modern software systems. Promising ways to overcome this are formal methods and metamodels, which should provude precision and provable consistency of system specifications, as well as the better understanding of the software development process through modeling the process itself. In this paper, a generic metamodel of object-oriented systems is specified using the Z formal notation. The metamodel is, then, applied towards derivation of complexity metrics, including the well-known Function Point measure and a recently introduced generic measure -- the System Meter. Both applications demonstrate the genericity of the approach.