Nick, I would speak to the Marine Officer who is assigned to your NROTC unit first. They would have the best contact information regarding the Officer Selection Officer (OSO) in your area and all the other information that you will need.
FYI - Almost all OSO's are assigned to Marine Recruiting STATIONS...not to be confused with substations which seem to be in just about every neighborhood these days.
You can find the recruiting stations here:
https://www.marines.usmc.mil/PA_NEW/MCRC_PA_STAGE_00.html
Platoon Leaders Class (PLC) is either two 6-week programs called PLC Juniors and Seniors or a single 10-week program called PLC Combined in which undergraduate college students attend OCS during their summer break. Candidates cannot enroll in PLC Seniors without succesfully completing the Juniors course. Upon successful completion of OCS, these officer candidates return to their respective colleges to finish their degree requirements before receiving a commission.
Officer Candidates Course (OCC) is a 10-week commissioning program for those who already hold a bachelor's degree from an accredited institution. OCC classes are held three times a year, in fall, winter, and summer.
All the above information was gotten at
http://www.ocs.usmc.mil/?dest=progs