Hehe! I was really worried about laughing fits at OCS also. I'm prone to extended, uncontrollable fits of laughter for random things. Naturally I thought for sure the staff would make me crack and laugh.
Turns out, OCS is full of stuff that isn't funny at all. You can bring those things to mind whenever you feel like you're going to snap. I didn't have much of a problem with it.
But, there was this one time before lights out, where you do this whole process of mounting the rack (PREPARE TO MOUNT!) and raising your arms straight up over your head, and then slapping them down on the rack three times, yelling, "HONOR, COURAGE, COMMITTMENT!"
Anyway, we all lined up on both sides of the racks, arms straight up in the air, waiting for the next command. There was this guy, who happened to be a decent man but was headed home for sure, and I guess he goofed something up that the platoon sergeant saw. He layed into him with the funniest stuff I'd heard since I arrived. Honestly I can't remember what he said, I think a lot of what made it funny was circumstance.
Anyway, I totally lost my sh!t. I couldn't stop laughing, and my rackmate started to lose his sh!t also. Tears were running down my face I was laughing so hard and trying to keep it silent, but we had to stand there with are arms straight up the whole time. I figured my rackmate and I were hosed for laughing, but then I looked down the line and saw two rows of arms, straight up in the air, all shaking with muted laughter. The platoon sergeant must have seen it, but I guess he let it slide that one time.
Finally we got in the racks, and lights went out. I spent a good five or ten minutes laughing to myself.
Anyway...don't worry about laughing, you'll be fine. Worry more about running and stuff, because that's what will make or break you.