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