The conceptual modeling of the Universe of Discourse (UoD) is an important phase for the development of information systems because the conceptual model is the basis for system development. Conceptual model specifications must be formal in order to be precise and unambiguous and to support consistency and completeness checks. The object-oriented paradigm is suitable for providing an integrated formal description of all relevant static and dynamic aspects of the UoD structured in objects. In this paper we introduce a formal concept of object suitable to represent the UoD by a collection of concurrent interacting objects. The \tt Oblog-language for object-oriented UoD-specification based on this concept supports the integrated description of data about objects, the development of objects through time and of various relationships between objects taking into account static and dynamic aspects of object interaction