That is now about to change as Oracle has released an Early Adopter Release of SQL Developer Data Modeling.  This is currently a standalone product but Oracle has said at some point it will be integrated with SQL Developer.

Oracle SQL Developer Data Modeling is the latest product offering to join the Oracle Database Tools. SQL Developer Data Modeling offers a full spectrum of data and database modeling tools and utilities, including Entity Relationship modeling, Relational (Database Design), Data Type and Multidimensional modeling, full forward and reverse engineering and code generation. It includes importing from and exporting to a variety of sources and targets, provides a variety of formatting options and validates the models through a predefined set of Design Rules.

Oracle SQL Developer Data Modeling

• Browse, edit, create database models using a Graphical User Interface
• Import from a script file or Oracle, IBM mainframe DB2, IBM DB2/UDB, Microsoft SQL Server and ODBC/JDBC database catalogs
• Import multidimensional Cube Views metadata and XMLA
• Import existing models from other modeling tools like CA ERwin® Data Modeler, Oracle Designer
• Printing facilities
• Synchronized forward and reverse engineering between Logical and Relational models
• Model compare and merge facilities
• Extensive formatting options and support for subviews
• Run on Linux, Mac OS X and Windows

Oracle already has a lot of documentation, tutorials and a FAQ online at the Data Modeling site.  If you are currently using SQL Developer I’d give it a try – even as an early release I’ve had good luck creating basic ERDs so far.