Your best two resources are the questions in the back of your Savarese book and the neuro section in your Simmons book. I took the OMM shelf over a year ago, and I remembered that it was hard like all of the other shelf exams, but I can't pinpoint what was so hard about it (most of it was probably attributable to the fact that it was one of my first shelf exams). I did well on it, though, so rest assured that my advice is sound 😀 From what I can remember, most of the test was neurology, some off the wall questions of course, and maybe one or two questions that have nothing to do with OMM at all. If you can get through all of the test questions in the back of your Savarese book at least once, then you are golden.
What helps with OMM shelf is the fact that you've been exposed to it for 2 years already and have already studied it extensively for your COMLEX I. Your OMM rotation is just a review of what you already know. Don't stress out about it. Best of luck!