This is my first time writing on the forums - however I have used it a lot throughout my recent studying - and I just wanted to give my insight into OCA prep for the exam (which I passed today).
Firstly I would just like to say that I only started learning Java 2 months ago - I am looking for a career change and this is my first stepping stone! But hopefully this will help others who are looking to do the same.
My main bit of advice for anyone looking to take the exam is - code, code and code some more. There is only so much that books and the internet can tell you, it is only through actually writing code that you truly understand what it means. Also it will give you the skills to be able to answer questions in the exam even if that particular method or syntax is new to you, simply by applying the general knowledge you have about the language.
That being said you will need a reference to study from. The book that I chose was the OCA book from Jeanne Boyarsky and Scott Selikoff. I can't sing it's praises enough - it covered absolutely everything I can came across in the exam and more. Also the way it was written made it very easy to understand new concepts in a way some other resources struggled to. The tests at the end of each chapter and also the mock exams are, in my opinion, much harder than the actual exam so passing these will give you much confidence. I could only recommend this book highly!!
I also made heavy use of the Java Online Docs and also just opening up the classes inside my IDE to see how they work. It definitely helped me understand why Strings are immutable by looking at the return of each method etc.
Finally if you are coding - steer away from IDE's and just use something simple like a text editor. I used Notepad++ as it at least adds some nice colours to the lettering. The reason for this as IDE's do a lot of the work for you in finding errors in your code before you try to compile - I found that if I tried to compile via the command line and an error or Exception occured I was much quicker to learn for the next time. We learn by our mistakes!
I am sure all of this advice has been written before but just wanted say that it isn't impossible and as long as you put the hours in (and enjoy it) you will have a great chance of succeeding!!
If you next challenge is the OCP 1Z0-809 exam then I suggest you look at https://coderanch.com/wiki/659976/Ocpjp-Wall-Fame.
You may find the OCP Wall Of Fame a valuable resource as you can find out how others prepared and what resources they used.
“The strongest of all warriors are these two — Time and Patience.” ― Leo Tolstoy, War and Peace