If you know you want to be an Officer of Marines, you need to contract with the OSO as soon as possible, they use to let you contract after 1 semester or if you were prior service upon entering college, but your OSO will give you specifics. The reason I say as soon as possible, is that if your just the run of the mill white guy, than your going to have allot of competition the longer you wait. The easiest route is go sign up with PLC early. After your junior year, your going to have a hard time of it, the OCC slots are hard to get and very competitive. The way the Marine Corps looks at it, the close to graduation applicants or just graduating college grads, are there because he realized he needs a job so that is why they are there, unlike the Freshman or Sophomore who by contracting early shows his desire to be a Marine is more than to just have a job.
If your wanting to be a reservist, many OSOs right now are less than willing to take reservists in because of the GWOT. Years ago it was the primary place to recruit future officers. However now, with the cycles of activations, etc they don't want to risk it.