The ODMG Standard is the only published standard for object data management systems. It con-sists of an Object Model for modeling applications, an Object Definition Language (ODL) for de-fining object database schemas to implement application models, and an Object Query Language (OQL) for writing queries to access object databases. We present a formal system to specify the data types that can be defined in an ODL database schema and analyze the data types required by OQL queries. The study indicates that OQL queries require data types beyond what can be de-fined in the ODL. It implies that, with the current specification of the ODMG Standard, no com-piler or interpreter could perform type checking for all the valid OQL queries. We extend the ODL type system with data types required by OQL queries.'