I used TopScore. I used all of the sections except for the Reading, so I can tell you how I thought all but RC stacked up against the real test. I thought that TopScore was much harder in all of the subjects I used it for. For physics, for example, AC circuits, which the OAT people specifically left off the list of possible questions, show up on TopScore. Also, TopScore required that you know every last equation, and the real thing only used the most basic equations. On the one hand, taking the TopScore practice tests worried me because I didn't seem to be doing as well as I wanted to, but I think it may have payed off in the end, because it made me study more because I was so worried I was going to do badly 🙂 . So, if you want to spend $50 to get yourself motivated, it might be a good idea. Otherwise, since it doesn't give you reasoning for why the correct answers are correct (it just tells you what topic the question fell under- like we couldn't tell that from reading the question!!😡 ), I would maybe get something else that acts as a practice test as well as a study guide that tells you more about the questions' topics. Good luck!