The SCJD / OCMJD is not an exam with question/answer mechanism. It is a project you do on your own computer and send for reviewing (you've got one year for this). The only common part with SCJP is that after you deveop the project, you are going to your Prometric test center and write an essay (about 4 questions) which are related to your project.
Just to complement what Pedro explained, the SCJD certification shows that you are able to create a software to solve problems from the real world, and thus, it is a practical certification. And then, you'll have the opportunity to architect and design your application using the patterns you find adequate and taking the decisions you find appropriate, considering the must requirements and other incomplete requirements.
Well, to me, it was the best certification I took. I learned a lot from it.