Oracle Enterprise Manager is a Java based GUI, and it allows you to review your tables, indexes, etc, but I don't think it draws pictures to show relationships. GUIs to view the relationships between tables might be provided in 3rd party tools, but there is nothing that comes free with Oracle server that does this, to the best of my knowledge.
Checkout Embarcadero Technologies. Maybe DB Artisan or Describe does what you want. If you have the Oracle 9iDS, then checkout Designer. It does this, but its a CASE tool and CM tool, and changes you make there can be implemented in the database, but it is not a tool for DBAs to use when implementing changes on the fly that don't need to be captured in the CM process.