(Mostly in reply to the original question)
I agree that the exam is MUCH more focused on recognizing correct and incorrect Java constructs/syntax than in learning how to properly implement business requirements in Java.
Yet it does teach you the fundamental nuts and bolts of the language. You do have to think more like a compiler than someone implementing a piece of business logic. Still, having that knowledge doesn't hurt and you don't have to give up your higher-level knowledge to learn it... I have years of experience writing Java code and still the exam questions (and especially mock exam questions) were challenging because they also test you on things that the compiler normally catches so that you fix it without a blink of the eye, some things you may never use in practice and some things you can quickly look up. Still I found it a nice experience to learn some of these details. It improves how I can analyze a piece of code and know what is legal and what is not, and being able to explain why this is so without having to guess. So I do have the feeling it made me a better programmer, though it won't hugely affect my performance on the job.
Others have made a valid point that passing the exam can also prove to others (including a possible employer) that you have a good grasp of the Java fundamentals, that you have a good attention to detail and can concentrate for a longer period of time.
As to retraining the brain: I scored 98% on the OCA, more than I could ever hope for! So I guess I did manage to do this... What I did is first going back to the basics. Even though I knew how to use many of the topics in practice, I still carefully read a study guide (the K&B book), as well as the Java tutorials listed by Oracle for the OCA
. This allowed me to pick up details I didn't know before. But more importantly: I made all of the practice questions and mock exam questions provided with the study guide, as well as bought and made the Enthuware mock exams. I focused on the mistakes I made and made notes on how to prevent these type of mistakes in the future. This is either memorizing a detailed piece of knowledge I had missed, or learning how to better tackle the exam questions, e.g. by checking for certain pitfalls. See also my tips on this post I made
Good luck with studying!