This paper proposes an extension of ODMG (Object Data Management Group) standard for the Object-Oriented Database Management Systems (OODBMS). The extension concentrates on composite objects, which provides a new paradigm, and also improves traditional OODBMS to meet the needs arising from the aggregation hierarchy. Currently in ODMG, the semantic of the aggregation relationship is explored at the modelling stage and is described in natural language. To formally specify and verify an aggregation relationship, the standardised ODL (Object Definition Language) and OIF (Object Interchange Format) must be utilised. Composite objects classes are defined in an extended ODL, and instance values are initialized with OIF. The proposed ODMG extension in this paper provides a formal foundation for the implementation of aggregation hierarchy in OODBMS.