You could think of OCS as a weeding-out process, and a seriously stressful one at that. Personally, I took a different route to my USMC commission (USNA), so I can't speak to the particulars. But I did do recruit training at Parris Island, and all the Sergeant Instructors at Quantico were Drill Instructors first. TBS is all about learning the basics of being a Marine Officer, to include uniforms, fundamental infantry tactics, structure of the Marine Corps, etc, etc, etc. There's quite a bit of field time and lots of land navigation practice, but it's nothing that thousands of other people haven't been able to handle. Besides, quite a lot of it is actually pretty damn fun, like throwing live grenades, firing various weapons, introduction to explosives, and even land nav (if you're into that sort of thing). Overall, TBS was a pretty good time for me, even though there were a few times when it genuinely sucked.
