About 15-20 percent is supposed to be OMM. Which is expected since it is D.O. board exam. Hope that helps! Path is supposed to be 50 percent, and 15 percent is supposed to be micro. The latter can depend. Good luck!
I see this statistic pretty often as well, though my school keeps quoting "30% of COMLEX is OMM" as though it's a fact. About 20% is certainly what the real thing felt like. The vast majority of questions will be very simple - OMM basics anyone could answer after their first semester in school. A minority of questions will be ridiculously obscure and specific - likely things you've never heard of before. Bottom line is I wouldn't sweat it. Focus on things you can actually study for.