Lambdas/Streams are obviously important because they are a big part of questions in other APIs (like Concurrency). Beyond that, you need to know everything reasonably well. Unlike the OCA exam, the OCP exam material is quite broad and varied.
In other words, questions aren't likely to repeat the same concept nearly as much. You need to understand a long list of topics (large and small) as you will encounter questions that certainly
test your retention. For example, there were some topics related to InputStream and
JDBC that Jeanne and I had never used, didn't think were important, and never thought they would be on the exam. Upon us both taking the beta/official exam, though, we were surprised to discover many very minor topics were on the exam, and updated our text to cover additional material. Aside from a few tidbits where we explicitly say a topic is not on the exam, you really do need to know everything in the book.
Make no mistake, though, the OCP exam is much harder than the OCA exam, in part due to the volume of material required to do well.