A guy in college with connections. The Big Man on Campus (or BMOC) knows people. He's privy to the locations of the best parties, the coolest hangout spots, and the loosest women. The term isn't used as much anymore due to the advent of internet social networking, allowing everyone to "know people"; but still, being a BMOC takes skill in real-world networking. BMOC's aren't always douchebags, but may have a slightly more inflated ego than most.
BMOC: "Ok, guys, I got us all invited to this sorority party tonight, and I know a liquor store we can stop at that doesn't card."
Other guy: "Whoa, check out the Big Man on Campus! Awesome, dude!"