Well, it become harder as you go further. But you shouldn't care of the complexity at all, as you are done with the basics and now harder topics are easy to understand for you. You may have experienced a slight difference between 1z0-007 and 1z0-031 that some topics that you have been covered in 1st exam may come with the advance options in the 2nd, like "Password management piques".
Furthermore, this is good to study the book twice. Because when you read a book again you realize that you skipped or haven't paid much attention to some more important topics, and it gives you a revision too.
I would like to state it again that your approach toward Oracle Certification (Prof./Mast) preparation is quit OK. But next two exams are bit harder. Both "Configuration" and "Architecture" are more complex topic. And it is completely different from what you have done in OCA. Unfortunately you can not take advantage of OCA preparation in OCP apart from that you qualify for the OCP. But the good habits you adopted during OCA preparing will take care of the OCP hardness. Like to read book again and again, to push yourself in a crowd of oracle professionals by using FAQ and discussion, to gather test/exam paper etc.
You should give more time to you study and research. And a secret about these two exams is to be practical in terms of client and servers side configuration.
Syed Mehar Raza
(Oracle Certified Master)