Object-oriented data base operators have been extensively studied in recent years. In this article we attempt to enhance the set of operators and define them in a more formal way based on graphs. We adopt directed acyclic graphs to model object-oriented data bases. The operators are defined by graph transformations. Based on this graph-theoretic approach, a family of operators on graphs are defined as the basic operators for object-oriented data bases. Moreover, many applications of graphs, such as matching problems, are used to enhance the set of operators on object-oriented data bases. We can use this kind of operator to implement certain queries that are adequate for new application domains such as decision support systems. We also develop a set of schema-restructuring operators which can be used to integrate individual schemas. The integration presents users with a logically integrated global view of the data stored in the individual schemas without requiring that the schemas be physically integrated. We use a query language based on SMALLTALK-like messages. Queries will be implemented by translating the queries into our defined operators, which are then interpreted.