It's hard to prepare without study guide, but here is my sources:
1) K&B OCA/OCP Java SE 7 Programmer I & II Study Guide (also used for OCA8)
2) OCA: Oracle Certified Associate Java SE 8 Programmer I Study Guide by Jeanne Boyarsky and Scott Selikoff
3) Enthuware Mock Exams 1Z0-809
I got the last one just before exam and passed only 2 practice tests and last day test (~55%).
I think, my score would have been better, if I had tried all the tests. It's a very good practice.
About the real exam questions difficulty:
A lot about streams and java.util.function (difficulty: from easy to tough and very tough)
A lot about Files, Paths, etc. (tough)
JDBC (easy / tough)
Localization and ResourceBundles (easy)
DateTime API (easy)
Fork/Join framework (very tough)
Advanced Class Design, Generics, Collections (easy / tough).