If it's done like it was in the good old days, academics is the most important contributor to OOM. In my day, it was about 70%, if memory serves. Thus, the other parts don't make a huge amount of difference, unless your marks are significantly higher or lower than your grades (i.e., if your academics put you in the C range and you have As in conduct and PE, etc., your OOM will go up a bit; the reverse is also true).
As Jadler03 says, OOM is very important. In my day, it was basically THE determining factor in service selection other than subs and even there it was a very important factor -- I believe that's not as much the case today.
One item of note . . . if you apply to grad school, etc., the rank on your transcript will be your academic class rank, not your OOM.