In my experience the Marine Corps notifies people when they are not selected by the board. After all, there are only two boards so it's not the same amount of effort and cost it would be for Navy, Army, etc. Also, remember that MO boards are at the district level so there may be some small variance in dates between them (I would think). The notifications after the second board for non-selects also includes information on how the applicant can still pursue a commission as a Marine Officer.
EDIT: It was several years ago now, but as I recall DS was informed via email both times. He also received snail mail after a negative reply from the second board.