You will need more than just the OG for study material since these are ONLY practice questions. You need to learn the knowledge behind how to approach questions, and how to understand the topics behind questions. I suggest that you purchase other/more study material (ex
Manhattan GMAT Guides). The guides use the OG as the practice questions. So keep studying hard. Don't worry about a lower score, it happens to all of us. The fact that you scored a 620 on the GMAT prep shows that you are starting at a strong spot. So keep up the hard work and you can get a 700+ for sure! I scored a 570 on my first practice test, and got a 710 9 weeks later.
Good luck!