There might be some difference, but it shouldn't be enough to throw anyone off guard. The best way to feel comfortable with the GMAT is to have perfect familiarity with all the concepts involved. This includes reading, and sometimes re-reading, good guidebooks. Another way is to practice with a sufficient amount of high-quality questions. Sources include the OG material, GMATPrep, Manhattan online CAT's and question banks,
GMATClub tests and questions on the forums, and Jeff Sackmann's math sets. While it's not necessary to use all of these options, a variety of reliable questions helps to solidify one's understanding of the fundamental concepts and progress to a more advanced level.
One common problem I notice is that many prospective test takers become too dependent on the OG problems. The OG is essential, but some of us just do the same problems over and over again. This is a mistake. By all means, practice with the OG and learn from your mistakes. But once the book is successfully completed, it's time to move on to other material. Continuing to do the OG after this point only makes one more passive, stale, and inflexible. These are characteristics that nobody wants to have on test day.