I went to Marine Corps OCS at Quantico, VA (Bulldog, Golf Company, Second Increment) in 1987. There are several “styles” of OCS, each with varying degrees of preparation. Mine was only six weeks at OCS, but I also had ROTC classes and drill every semester in college, and a month each summer on active training, for added training and evaluation.
In a nutshell, Basic Training (Boot Camp) breaks you down emotionally and psychologically, then builds you back up as a functioning member of a unit, and as a person you didn’t know you could be.
OCS is more of a screening process, weeding out those who don’t have what it takes to be a leader. There certainly is that aspect of breaking you down, but I don’t believe the emphasis is quite so much on that as it is on performing and making sound decisions under duress.
It is extremely physical, emphasis on endurance and upper body strength. Classes are not academically challenging, but they are hard to stay awake in, which is a challenge in and of itself.
In 1987, they did not give you enough time to do everything you had to do (at least for 90% of the Candidates), and you had to prioritize your uniform, rack, weapon, and person appearance for inspections. Lights out were at a given time, but most people stayed up afterwards working on something for the next day. That may have changed since then.
I lived in a squad bay just like they have on Parris Island (very similar, at least). I think we had less time to eat chow than enlisted Recruits get, but I am not sure. There was more of a push toward leadership at OCS than Boot Camp. I had many Enlisted Marines in my platoon (my rack mate was a Sergeant, guy next to me a Corporal, one guy was actually promoted to Gunnery Sergeant during OCS) so they talked about the differences they saw, but I have never been through Boot Camp, so all I know is second hand info.
Marine Corps OCS Instructors (Staff Sergeants, Gunnery Sergeants, and Sergeants Major) are all Drill Instructors with at least one tour on the drill field, so they are practiced and professional yellers and screamers, and as Drill Instructors, they are the cream of the crop.