Well generally Kaplan tests and the study materials will be more difficult than your actual oat. The books and test are designed to get you thinking in depth about the concepts and details of questions. Just be careful. The actual OAT was more general than the kaplan which threw me a bit.
Defnitely harder. Harder b/c they want you to see you need "help" and entice you to take their class. And harder b/c all of their practice tests are more difficult.
I really benefited from the Kaplan material--- I am a non-trad, out of school for 2+ yrs. I did well on my first try and since I was used to the super-hard Kaplan questions/practice tests, once I sat down at the real OAT and started answering questions.... it was the BEST feeling thinking, "wow, this is really not that bad!" Got that feeling w/in 5 mins. What a relief.
I just finished with the Kaplan online class. All the practice tests are hard Get used to it and then the oat will be a breeze. On my actual OAT, after my first 4-5 physics questions, I had to sit back and say..." are you kidding me?". That's the type of situation you want to be in. Fortuantely, you have the time to get a free practice test in. Have fun.