A specification language is important in rapid prototyping area in software development environment. In this paper, we explain a computational model OOAG. Using this model, we can execute the specification just written in OOAG based language. OOAG is a model based on attribute grammars and extended by object oriented concepts for describing system's dynamic behavior. We show the ability of OOAG for describing structure oriented systems, by describing UNIX file system using OOAG.