If remember correctly, those who attend USMMA who earn USMC billets, must attend OCS. Which program of OCS, not 100%. I am sure one of the KP alum or Mids will know.
USMCGrunt, Leatherneck is really modeled after TBS, not OCS. It is a training environment very similar to Leatherneck. Focuses on USMC traditions/operations/familiarization, land nav, small unit tactics, weapons familiarization, PT, etc. They actually use quiet a few of the training POIs from TBS for Leatherneck.