They have a trial version or a full version for ~10$.
I am not sure there are any free full featured websites providing OCA training.
If you buy/own the book "OCA: Oracle® Certified Associate Java® SE 8 Programmer I Study Guide" by Jeanne Boyarsky and Scott Selikoff you get access to http://sybextestbanks.wiley.com, where you will have an electronic version of the review questions from the book, and 3 practice exams.
There is also this second book "OCA Java® SE 8 Programmer I Exam Guide" by Kathy Sierra and Bert Bates which grants access to some sort of electronic version of the test. I don't know exactly how, because I am just starting this one...
P.S. I am also studying for the OCA right now, and I use 2 electronic book and 2 hard copy books.
P.P.S. Good luck.
I don't recommend this if you are a beginner, but, if you want and feeling extra resourceful, you can try learning through the ultimate source, the language specification: https://docs.oracle.com/javase/specs/jls/se8. Be warned though, this is not beginner friendly.