The conceptual modeling of the Universe of Discourse (UoD) is the most important phase for the development of databases and information systems because the conceptual model forms the basis for system development. In this paper we introduce the \tt Oblog-language for object-oriented specification of information systems. \tt Oblog supports a complete representation of the relevant aspects of the UoD by integrated description of data about objects, of it's manipulation, of the development of objects through time and of various relationships between objects. The semantics of the language is defined using a rigorous mathematical object model