the help within Oracle Designer is extremely comprehensive and covers what you are after. Just do a search for Design Capture and entity retrofit.
Essentially you need to reverse engineer the tables and, then produce a schema diagram within Designer. If you want to produce an ERD, you need to take the tables back to entities via the table to entity retrofit utility
This article explains and demonstrates how to Capture Design a Server Model from a Database. It is intended for entry level Designer programmers and applies to Designer 10G version 18.104.22.168.
How to Capture Design into a Designer Repository
The following is a procedure and demonstration of how to Capture Design any part of the Database into Designer such as Tables, Views, Triggers, etc.
1. From Designers Front Panel select Design Editor.
2. Select the Application you are trying to capture into from the Navigator. In the example, we used Scott_Tiger_Capture_Design application.
3. Determine that the objects you are trying to capture do not already exist.
4. From the Tool bar: Generate - Capture Design of - Server Model.
5. Select the Database and fill in the Username password and connect string for the database that you are trying to capture.
6. Select the Database and User to place the objects into the repository.
7. Select the Objects tab at the top
8. Move the objects you are trying to capture from the Left to the right. You may select one, some, or all objects to be captured.
10. During the Capture, the object names will be Blue and turn to black when the capture design is complete.
11. There may be some warnings as shown in the example. You will need to review the warnings and determine if a course of action is necessary.
12. Warnings will be in Red and must be resolved before the capture can be completed.
How to reverse engineer an ERD in Designer
The following is a procedure and demonstration of how to How to reverse engineer an ERD in Designer
1. If you do not have RTD's in the DE then refer to Note 260617.1 on how to Capture Design tables from a Database.
2. Select Entity Relationship Diagrammer from the front panel of Designer.
3. Select File - New Use the drop down list to select the Application or the text button and select the Application. This will open a new diagram area.
4. Utilities - Table to Entity Retofit This will open the Table to Entity Retofit screen a. Select Candiate Tables button. Select the tables you want to make entities. b. Select Retrofit The tables will generate into entites. c. Close both screens when done.
5. Edit - Include - Entity Select the Entities you want to include in your diagram. - OK