I first made sure that my previous certifications were showing up on certview - https://certview.oracle.com/
I found that I had two identities that I had to merge - one from Sun Microsystems and another from signing up with Oracle. The help desk sorted that out - that was about a year ago.
I then read the OCA/OCP books by Boyarsky & Selikoff, available on Amazon - https://www.amazon.com/OCA-OCP-Java-Programmer-Certification/dp/1119272092
The OCA was mostly revision with a starter on lambdas. The OCP book gave a good foundation for the exam - leaving out chapters 8 (java.io streams) and 10(JDBC).
One thing missing from the books is PathMatcher but this is easy to cover on your own. Don't forget it as there were about 2 questions on my exam that included it.
I then drew about a dozen mindmaps based on the chapters to categorise what was in the OCP book. The mindmap for NIO.2 was the most complex (and this was the section I kept failing). The next hardest was streams but it's very mathematical and I enjoyed it.
Finally, I bought the pack of tests from Whizlabs and that added a number of nuances that were not in the books or the final exam but which helped expand my horizon and confidence. About 10 questions out of 60 in each practice exam covered things that were not in the book.