The questions you face on the test will generally be clustered around whatever level you're scoring at on the day you take the test (though you'll obviously have some higher ones and some lower ones). OG11 provides a range of questions, from low to high difficulty. As such, there's no way to say, in general, how difficult the OG is compared to the exam. If you are a high scorer, a higher proprotion of the problems in OG will be easier than the ones you get on the test. If you are a lower scorer, a higher proportion of the problems in OG will be harder than the ones you get on the test.

It's a good sign that you're generally feeling more comfortable with OG questions - you want that to continue. Take a practice test to get a sense of the levels at which you're scoring right now and see how that score compares to where you'd like to be. If you're at, say, a 600, and you want a 700, then you need to do a couple of things:

1) make sure that you can answer everything below a 600 level without going over 2 min - you don't make careless mistakes or otherwise lose points below that level

2) figure out how you can get even more efficient with those lower level problems - how can you do them in 1.5 minutes instead of 2? You'll still get some lower level problems, and you want to do them both correctly and efficiently so you have enough time for the even harder problems you're (hopefully!) going to get.

3) start to build on your base - don't jump right to 700+ level problems. Work on getting good at problems in the 600-700 range. What's the difference between and exponent problem at the 500 level vs. 600 vs. 650? What additional content do you need to know? How do the problems get trickier? How do you need to execute in order to do these harder problems (in the same content areas) more efficiently and effectively? And so on.

If you want a 700, don't stress about mastering 700+ level problems. Theoretically speaking, you could get every 700 level problem wrong and still get a 700 - as long as you get all the sub-700 problems right. Things won't work out exactly that way of course - but the point is, you won't get offered many 700+ problems if you aren't doing well with the 600-700 level problems, so you've got to make sure you get those first. Plus, the 700+ level problems are essentially bonuses - get something right and it could help you but get it wrong and it won't hurt much (again, assuming you're going for a 700 score - if you want a score, the same principle applies for whatever that score is).

So go take that practice test and analyze your strengths and weaknesses across (a) question type (and sub-type, if applicable), (b) content area, and (c) timing. Then set up an appropriate study plan and go!

p.s. ideally, take a practice test from a test prep company at this point, not GMATPrep. GMATPrep is great in that it's the closest thing to the real test, but it's not so great in that it gives you no analysis to work with. You need that analysis to set up and adjust your study plan over time.