I prepared for 3 months with resources suggested by other lucky certificated developers in this forum.
I spent about 2-3 hours a day for books and tests. I took all chapter tests and after finishing the book - took mock exams.
Books: Oracle Certified Associate Java SE 8 Programmer I Study Guide: Exam 1Z0-808 by Jeanne Boyarsky, Scott Selikoff
OCA Java SE 8 Programmer I Certification Guide by Mala Gupta