This is taken from one of my posts in another thread about a similar topic:
Alright, I'll lay out the different Marine Corps commissioning sources:
USNA: Assuming you are accepted and get through Plebe year, you will elect to go to Leatherneck between 2/c and 1/c years. This is a 4-week evaluation/familiarization period at MCB Quantico for Marine-select hopefuls. At this point, if you don't do well at Leatherneck, you will not be selected as a Marine officer. After commissioning, you will go to The Basic School for six months. Near the end of TBS, you will select your MOS. This is the only way to become a Marine Corps officer without going to OCS.
NROTC: You apply for a Marine Option scholarship. The purpose of your first three years in the program is to prepare you for a six-week session at Marine OCS (technically called "Bulldog", but it is identical to the Seniors portion of the PLC program). If you graduate from OCS, you will have another year of NROTC to get your degree/commission, and then you will go back to Quantico for TBS. After this, the track is identical to USNA.
PLC: Comes in two flavors, "Juniors/Seniors" and "Combined". J/S is two six-week trips to OCS in Quantico. The name is misleading, as you can actually do Juniors after your freshman year of college. Seniors will always be immediately before your senior year. You will commission after you graduate and then go to TBS.
"Combined" is a ten-week program that takes place immediately before your senior year. After you graduate, your track is identical to Juniors/Seniors.
OCC: Identical to PLC-Combined, but you must have a college degree or be a college senior to apply.
The difference with PLC and OCC to NROTC/USNA is that when you apply to PLC or OCC, you apply for an Air, Ground, or Law contract. This means that, for example, if you apply for and receive an Air contract, you will go to flight school after TBS, assuming that you remain physically qualified.
Note that, in all cases, you must have a bachelor's degree to become a Marine officer. There is no way to go from enlisted to officer without said degree.