I did OCP 8 today and must say that yes, gap between OCA and OCP is really big. From my point of view there is not so big problem with hard questions - where you can use logic or intuition but the problem is in questions about behavior of specific methods or classes where you normally never used them but you must know how they work. So you should
know well really wide spectrum from Java APIs (Stream API, IO, NIO2, JDBC
, Concurrency, L10n, ... - lot of classes with lot of methods).
I started to use Java 8 in work this year on one project, unfortunately other projects are still on Java 7 and 6. And I must say that I feel that 2 months of working with Java 8 saved me during today exam because I started to love Stream API and approximately one third of all questions was about Stream API ;-)
From my experience, doing OCP in short term after OCA is also good because you have fresh experience of tricks they use and you will remember better how work some parts which will be in OCP also included (access modifiers, overloading and overriding, casting and so on).
At the end my OCP passing score was 5% worse than OCA passing score ...