This is a pretty good data modelling tool that allows you to import an existing model from a database schema or from other tools e.g. Oracle Designer. You can then view and maintain the model easily via the diagramming tools.
It's easy to install and get started with, costs nothing, and it provides a lot of functionality for data modelling, DDL generation and so on.
No more Blub for me, thank you, Vicar.
subject: Oracle Database showing existing tables and their relationships .