I am sorry to disagree. MANY of the OG questions are challenging and if you are seeing them for the first time, getting the answer right (FROM THE CHOICES) in under 2 minutes is NO JOKE. GMAT questions MIRROR OG. As Aka says, it is the only book that is almost perfectly balanced. Kaplan comes WAY behind OG although Kaplan is way ahead of all the other stuff that is pedalled.
Powerprep has questions from OG. But if you have EVER seen or done the questions from OG, your powerprep experience is futile and the scores that it represents are not a correct representation of your GMAT. ONLY if you have never seen the OG - AND you do attempt the powerprep - then yes, your PP scores will be very near to what you will get in the actual test.
All of the above - whatever I have written - and more - has been put in this forum many times. I would request that new posters take the time to go through previous posts. This is an invaluable and incomparable resource.