I work full-time as well. I've been preparing myself for the OCA exam since April, 17, I guess. I scheduled to take the exam on July, 3, which means next monday
I studied with the following books: OCA Java SE 8 Study Guide (Jeanne Boyarsky and Scott Selikoff), OCA/OCP Java SE 7 Study Guide (Katy Sierra and Bert Bates), and Core Java Vol. I (Cay Horstmann and Gary Cornell).
I studied 1 chapter per week (from the study guide by Jeanne and Scott), following this sequence:
read the chapter;practiced with a lot of code;read about the subject in the other two books that I mentioned;took the chapter exam;created mind maps and flashcards.
Then, I started to take the mock exams from the study guide, to see what I should improve, and studied again chapters 3, 4 and 5 from the book, studied my flashcards, studied my notes/mind maps (about all the subjects), and practiced again.
After all this, I started to take Enthuware mock exams, and I've been doing it for 2 weeks, and I'll finish the mock exams next weekend.
And of course, between my studies, I created and read a lot of topics here in the ranch, and read about some concepts in the Java Language Specification.
Sometimes I stayed up until 2 a.m. to accomplish my tasks, even knowing that I have to wake up around 6 a.m. It's very exhausting.
I hope I will be here on monday, with a detailed topic about my success on the exam