Newmei,
Are you talking about the OPM or the AT-SAT? I've taken and passed the OPM and found it to be effectively a joke. If you have more than a few brain cells functioning, you'll likely pass. I have not personally taken the AT-SAT, but a friend of mine took that test and said it seemed harder than it actually was. He passed with an excellent score (mid nineties). I will note that the OPM is pass/fail only, you don't get to know your score.
There are study guides available for the OPM. I used one from ASA, a dark green book called "Air Traffic Control Test Prep". It may not be at your local pilot shop but you can order it at ASA's site or at Amazon.
I don't have pass/fail stats for these tests, sorry. I will tell you that when my group of students took the OPM last year, only one failed out of 55...same this year.
Any questions, feel free to PM or email.
Stephanie