Hi minniemba.
I think you have gotten some pretty good answers so far, but I want to add this.
The GMAT tests skill in getting to answers, and an aspect of that testing is that while the questions you see on the test will at times be somewhat like the ones that you have already seen in the OG, they will not be almost exactly like them. In other words, while you may see certain types of patterns repeated, just knowing how to do the questions in the OG is not sufficient for scoring high on the test.
Further, the harder questions in the OG are not really as challenging as the most challenging questions on the test.
Here is something that helps to clarify what I am getting at.
I know of multiple people who not only have done every OG question but also have taken every official practice test multiple times and in so doing have gotten so familiar with all of the official questions made available by GMAC that these people score well into the 700's on GMAT Prep tests that they take. The people I am talking about don't just know the answers to many many official questions. They can explain how to arrive at those answers. These same people, people who have been preparing diligently and for extended periods of time, go to take the GMAT and score below average, i.e. 500's on the real test, versus maybe 780 on GMAT Prep.
Why does this happen? Because the people I am talking about study by reading explanations and learning how to do every available question without ever really learning how to ON THEIR OWN hack their ways to correct answers.
Get it?
You can work on every question and read every explanation to the point where you know EXACTLY how to answer every released official question around, but if the process you are using in your preparation does not result in your developing skill in taking information provided in GMAT questions and using it to work your way to answers, then you won't really have accomplished that which you have to in order to score high on the test.
In other words, reading and remembering someone else's explanations, even hundreds or thousands of such explanations, will only get you so far. The GMAT does not really test to see whether you know how to answer questions just like the ones in the official guide. Unlike many academic tests, which include questions that are almost duplicates of those seen in the course materials, the GMAT is full of questions that, while they have things in common with questions you may have seen, are somehow unique. So, to score high on the GMAT, you need to be skilled in figuring out ways to get to the answers to questions not quite like any you have seen before, and preparing for the GMAT must result in developing that skill if that preparation is to be effective.
The GMAT tests things like skill in seeing details, skill in making sense of arguments and skill in using available resources to make decisions and arrive at solutions. So in some way your preparation has to result in development of skills like those.
