In this paper, we propose an integration of an extension of Bailin's object-oriented requirements specification (OOS) with a formal notation (Z), called OOSZ. In OOSZ, the extended OOS is used to guide the derivation of Z specifications. A function process is converted into an operation schema. An entity process is manifested through a state schema, an instance creation operation schema and an abstract operation schema in the Z specifications. These are illustrated using the problem domain of Auto Banking Systems. The bringing together of diagrammatical and text elements of OOS specifications in Z notations offers three major benefits. First, OOSZ provides a systematic approach to producing a requirements specification which is both structuredand formal. Second, the OOS can be used to aid the analyst's understanding of the system for developing the Z specifications. Third, the OOS specification is improved by the precision of Z and through insights into the system gained during formalization.