Since the GMAT is a computer adaptive test, it's not easy to compare it to paper linear study materials.
The OG, for example, has a mix of easy, medium and difficult questions (most of the questions in the guide are medium level difficulty). So, if you're a mid-500s scorer, then the questions in the OG will be very representative of the questions that you see on test day.
On the other hand, if you're a mid-600s+ scorer, the vast majority of the questions in the OG will be easier than what you see on test day. Most of the people who say that their real GMAT was tougher than the OG fall into this category.
So, don't worry too much about OG vs Test Day. Make sure that you master the material covered by the OG and for higher level practice rely on accredited CATs (e.g. GMAT prep and the ones offered by major companies such as Kaplan) and study materials like the Kaplan 800 study guide.
Finally, don't jump into the high level questions too quickly. One major mistake that people make is to focus only on the really tough questions, not realizing that to get to the hard stuff, you need to get through the medium level questions that appear at the beginning of each section. If you only do high end questions then you haven't been exposed to representative medium level ones and often struggle with the first 4-6 questions that you see, throwing off your entire test.
