In this paper we present an Object-Oriented (OO) methodology for software production, and we focus on the analysis to design transition. OO-METHOD provides software tools covering the classical development phases of analysis, design and implementation, all this within a graphical, user-friendly working environment. Starting from a graphical system specification as the product of the OO analysis step, we produce the corresponding system specification in OASIS (a formal and OO specification language used as design tool). Finally, depending on the chosen working environment (logic or OO), we can obtain a final software product automatically by using the corresponding translator.