Last week I passed the Java 8 OCP with 90%. This is how I did it.
I started with reading chapter 1 to chapter 10 of Boyarski/Selikoff's book OCP Oracle Certified Professional Java SE8 Programmer II Study guide, making sure I understood everything, marking key points per section for next study-iterations. Also did the review questions for all chapters along the way. I did this during two weeks of holidays.
The next iteration was about 6 months later, due to heavy work load at work (not ideal, as things had faded a good deal). This time focus on memorization and exercises. I remade all the exercises from the book and did a lot of extra exercises from Enthuware test bank, per topic as I was progressing in the book. Did this during the weekends, evenings and some holidays, for a lead time of 3 months. To make sure I didn't forget what was in chapter 1 till I got at chapter 10, I kept on doing Enthuware exercises for all chapters I already did during these 3 months.
During the last month before the exam, I did a final iteration of the book, and did 5 Enthuware mock exams (results: 79%, 68%, 75%, 86%, 81%), doing a thorough review of all the questions after completing an exam.
The exam is 85 questions which need to be completed in 150 minutes. I always tried to do 20 questions per 30 minutes, giving me 20 minutes of buffer at the end to do questions that were more time consuming, which I marked along the way.
@Simon, other study material I used were the answers that are provided in the Enthuware software. It's very useful to read these (and sometimes there is a bit more info, like for enums). For Java IO, I used the http://www.falkhausen.de/Java-8/ website to see the structure better of the java.io package and which methods there are per class (e.g. Writer has append methods, and consequently PrintWriter, while OutputStream does not have append methods, but PrintStream does have append methods anyway). This website is also handy for other packages.