We present a concept of type derivation in order to introduce external models in object oriented database systems. This leads to the traditional three level DBMS architecture, consisting of an internal, a conceptual and several external models. In contrast to other approaches, our concept takes into account all traditional features of external models - such as submodeling, interfacing application programs and databases, logical data independence, canned queries, and individualized access and security management. We provide a clear separation of the type and class hierarchies of the external schema from those of the conceptual schema. This approach allows for better and cleaner modularization of information systems built on top of object-oriented databases. In this paper, we concentrate on the mechanisms to separate and connect the external and conceptual type hierarchies.

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