You'll take about fifty percent of the first year DO classes, starting second semester. I'm not sure what the postbacc people do in their first semester--I think it is mostly cell bio, genetics, etc. classes at the graduate level--but just about all of the second semester is the same schedule as it is for the first years, save for the last class, Mechanisms of Disease. The program is pretty intense, with many of the students doing better on the tests than the first years, mostly because the cut-off point for acceptance is an 85% (acceptance still not guaranteed) whereas the class average for first year classes (for the med students) is scaled to 83%. With that said, whatever cell bio, genetics, etc. you take would probably help, even though it would be largely redundant. It wouldn't hurt to pick up a Netter your first semester and go through it in your free time, as you'll be in the anatomy lab with the first years during the second semester, taking the same practicals as they do.
There are four or five people in our class from the postbacc program. I think there were ten or so people in the program when they were enrolled so quite a few don't get in, but don't quote me on that.