Complexity of real-life systems requires the implementation phase to be preceded by analysis and design. A model employed by a method, to be useful, should be complete, abstract, unambiguous and coherent. Unfortunately, most object oriented analysis and design methods, being informal, fall far short of the ideal. In this paper, we examine one of such methods, the OMT method of Rumbaugh et al. We point out the shortcomings of the method which would adversely affect the system development process. We then outline a practical procedure which shows how to proceed from the initial informal description to a final model formally specified in Object-Z. We also point out the advantages of the Object-Z model for subsequent phases of system development.