Not QUITE a Mustang just yet (5/7 is my big day), but I have been in for ~8 years.
MCRD and Quantico are yin/yang. Boot camp is designed to produce a basically trained Marine. Life was structured, every hour was accounted for, and the DIs ran the show. The WHOLE show.
OCS, as 1812 stated, is to "screen and evaluate." EVERYTHING you do, or fail to do, is noted. Hence the Spear Evals..oops, I meant "Peer Evals." Since your peers are with you 24/7, they act as ~12 other sets of eyes, in addition to your platoon staff. And, believe it or not, other candidates are very candid. (sorry...) If you are slacking off, they will nail you on it. If you piss vinegar, they will say so. It is not quite as structured as boot camp, as YOU run the show. The SI's pretty much just sit back, evaluate, and try to stress the gel out of you. Also, the black flag days jack the training schedule up pretty badly.
Your "free time" is ususally reserved for after Taps; even though you are officially supposed to be in the rack, it is the only time you usually have to prep your uniforms, gear, write your obligatory essays, etc. If your candidate staff is squared away, things will get done quicker, and the platoon sergeant might give you some free time. Basically, to make a long story longer, you are not "guaranteed" F.O.T. like you are at the MCRDs.
OCS evaluations affect OCS standing, specifically your leadership grade. They are also a general evaluation tool - to track your progress, and to have a nice paper trail, should the need arise. If all of your peers rank you dead last, you might spend your Saturday on an evaluation board. Other than that, evals stay at OCS, and have nothing to do with TBS or your fitreps. (Speaking of fitreps, you will receive a "Not Observed" report, but since you are a corporal, don't worry about it.)