How do you feel about your knowledge of the material? Forget the practice test scores and ask yourself how confident you feel in these subjects. Okay, you got 280's on your subject exams, and you're probably thinking its a good predictor of your real OAT, therefore you believe you don't know the material that well otherwise you would have gotten high scores. Forget all that for a second.
Let's say you're on Jeopardy, and these subjects (QR, biology, physics, etc) were categories. Would you think to yourself "YES, this is my dream board," or "Damn, despite having all of these in my curriculum at college, especially since I majored and minored in half of them, I would have rather preferred Potent Potables."
Are you at conflict because you feel you know everything but end up getting low marks or are you walking in not knowing topics but just hoping to do well.
I bet you're studying too, reading and going through flashcards. Are you learning or memorizing? If I told you to write as much as you can about Sn1 Reactions, how long would your paper be. What If I told you to write as much as you can about Sn1 Reactions in only 2 minutes?
Is your greatest weakness the material or time?
If you feel good but just aren't getting the numbers you think you're capable of gettin on these practice tests, then don't stress too much because you'll probably score higher on the real thing. Why do I believe on that assumption? When I took the practice OAT, it felt like a big sudoku, where you want to put a 9 in the corner but it just doesn't work out. When I took the real OAT, it felt like a teaser, saying "hey buddy, the answers right here, just think about it for a sec and it'll reveal itself." In other words, the real OAT was more general and should set off many lightbulbs from your past experiences in the curriculum. Or at least that is how my exam was for me.
In retrospect, this post is mostly nonsense. If it does you harm, sorry.