I have the OCA Java 8 cert. I remember studying for it and really getting familiar with evaluating Java code in my head. About a week later I had a Java interview and got the job.
At that job there was a java enterprise app written by consultants with PhDs in math and CS who were not around to enhance the application. There was also no JavaDoc comments to speak of - no public classes or methods had documentation.
I was the only one on the team who had a Java cert so was given the task to enhance it with the new requirements. I did what I could and started reading. It was really challenging at times but after about 2 months, I was able to create enough of a mental model of the codebase, and how to use the classes and interfaces, to make the enhancement.
The OCA certainly made me familiar enough with Java such that syntax, and following along with code blocks was not a hurdle for me anymore. The main challenge was the semantics, the domain modeling, and design patterns which I had to really struggle to understand the how and why of it all. Those things the cert will not teach.