Not being a Marine, I might be speaking out of turn; however, knowing several Marines @ VMI, it is my understanding that MOS assignment depends on how well you do at TBS. All new 2LTs have to attend it, regardless of source of commission. Thus, while USNA people might not have to attend OCS, they do attend TBS with people from ROTC/PLC etc. If there is someone out there with USMC credentials who would care to correct me, please go ahead.