I took the kaplan class and the free tests they provide online. I took the OAT two times.
The first time I took the OAT, I relied only on kaplan. When it came to the actual OAT, I felt really unprepared for the biology section. The questions on the biology section seemed more specific then the kaplan tests. I scored 340 in biology on all the kaplan practice tests I took and 290 on the actual oat.
Second time I took the OAT, I bought the oat destroyer which has hundreds of questions in every section which are completely different then what you will see on kaplan tests. The questions in the OAT destroyer are harder but if you can answer those then the OAT will be a piece of cake for you. I felt way more prepared and scored 350 on the biology section and all my scores went up by 20-30 points.
I think you have to review more notes and questions outside of kaplan to feel fully prepared for the oat. I still recommend you buy the kaplan review notes or take a kaplan class. To answer your question, I think the kaplan oat is easier than the real oat, and personally for me did not seem to be an accurate measure.
One more thing, I registered for a kaplan of the free tests over the summer and they all had the same question.