ER diagram from an existing ORACLE DB

In SQL Developer 3, this is very straight forward;

View > Data Modeler > Browser.

The browser will show up as one of the tabs along the left-hand side.
Click on the Browser tab, expand the design, right-click Relational Models and select 'New Relational Model'.
Then just drag the tables you want onto the model.

To save/export the diagram to a PDF do;

File > Data Modeler > Print Diagram > PDF

Follow instructions to finish

happy modelling