Actually, at the end of your OW class (and after you have completed the required dives) you are a Certified diver. You can now rent gear and go diving. That doesn't mean you're a great diver after your OW certification, just a diver.
Each certification agency has different courses that you can use to improve your skills. Many are focussed on particular diving specialities (caving, wreck diving, etc.).
If you specify an Agency you're planning on getting your training from, and where you're located, various people here on the board would be able to offer more detailed information.