Lets see....I didn't go to PI, but I think SD will suffice. OCS and boot are two different animals. I was only 18 (barely) when I went to boot camp and that was 9 1/2 years ago, so it's a little blury. But as long as you can follow orders, move with a sense of urgency, do 3 pullups, a couple dozen crunchs, and run a 28 min 3-mile, you can probably survice boot camp.
OCS...the first thing that comes to mind is the physical aspect. You run everyday at OCS, and a good run at that. I recommend being able to run at least a 20 or 21, 15+ pullups and Max crunchs. If you can't you'll have a hard time, and if you make it you'll probably be close to maxing the PFT at the end. The Mental part of OCS wasn't as tough as the Boot camp stuff, but that's my opinion, other priors may say otherwise. Anyway, the first couple of weeks is more like Boot, but after that they (Sgt Inst's & Plt Sgt) back off and let the candidates run the show. If you can remember back to boot, the DI's told you when to fall out for chow, classes, etc..., at OCS the candidate staff is responsible for getting his/her Plt to the assigned event, early! Once stuff starts getting F'ed up, the Sgt Inst's will intervene. Plus, you'll have multible billets, from Fire team leader to Candidate Company Commander, so be ready to take responsibility for other candidates mistakes. The hardest thing to deal with (for me) was sleep deprivation. In boot you got a Manditory 7-8 hours, and OCS you are lucky to get 4-5 a night.
Oh yeah, I can't leave out the Liberty you'll recieve the third weekend.
I'm sure other Mustangs can add to this...
Beetle