You Attend OCS because you will recievie training on your military bearing, and general information to help you start your military service as an officer. Those who served on active duty for years, and selected for STA-21 shouldnt have to do OCS because they already have military bearing etc, the Newport RI program is a rigourous physical conditioning program and to help math and science skills and gives prior enlisteds a framework of what being an officer is about, but for a civilian who had no ROTC and wants to join the military , comes your purpose for an OCS, and even after then you still need 1-3 years of learning from chiefs and petty officers and other officers to become profiencent.