How to export an ER diagram from an Oracle DB?

Have you got access to Microsoft Visio?

If you can get an ODBC connection to your Oracle DB then you can use Visio to scan the database. It will display all the entities, relationships, etc.

Have a look at this on Stack Exchange https://stackoverflow.com/questions/4891387/how-can-i-import-the-contents-of-an-oracle-database-into-visio-to-create-an-enti

Hope that helps

Oracle has a free tool SQL Developer that has can presumably reverse engineer an Oracle database.

If the database isn't too large then you could possibly use a tool like DbVisualizer (which is either free or low cost depending on the version). You can only diagram one schema at a time and results aren't directly editible (it's really more of an exploration tool) but you can save the diagram as GML and edit it with a tool such as yEd (which is a free download) (Note that you will need to edit the gml file slightly before bringing it into yEd by replacing all instances of 'customconfiguration "SimpleRectangle"' with 'type "rectangle"').

Note that SQL Developer, DbVisualizer, and yEd are all cross platform tools so you can use them on any system that has Java installed.

Update -- I just tried reverse engineering (154 tables) using SQL Developer. It appears to work reasonably well but it isn't going to win any beauty contests...

There's an open source project in SourceForge called Schema Spy (http://schemaspy.sourceforge.net/) which generates HTML schema reports, including ERDs: http://schemaspy.sourceforge.net/unifieddb/mangos/relationships.html

On the good side this tool is free, however the diagrams are not editable.