Will I gain weight in OSUT?

Damn which is more important to you getting thru OSUT or looking buff?

Focus on your fitness and do not focus on gaining weight. In the Army, you can put yourself in hot water if you get too heavy (flagged for failing body composition standards, etc.) despite being "fit" and able to pass the APFT. I've seen friends go through hell because they were too heavy and I do not wish that stress on anybody.

I went to OSUT back in 2011, and I was much like you: 5'10", ~145lbs, and very lean. I had played football as a DB, and was a sprinter and pole vaulter who spent a ton of time in the weight room.

And I got fat at Infantry School.

My Drills put me on double rations because I couldn't afford to lose weight without ending up being underweight by Army height/weight standards. I gained something close to 30lbs in those 14 weeks, and it wasn't muscle.

I'm not saying that will necessarily happen to you, but it did happen to me. My activity level throughout high school was very high, with football practice, morning workouts, track workouts, and my work pouring concrete while in the DEP.

OSUT was more stressful than those, but it wasn't physically more challenging for me.

Lots of guys lose weight there, and some, like me, gain a lot of weight there. The best I can say is that, if you see negative body changes and have the energy in your limited downtime, you can do extra PT in the barracks. Some of the guys in my platoon did, including me, but we didn't start doing that until the last few weeks. Mostly just extra pushups, dips using the bunks, various core exercises, and pullups on the bathroom crossbars.

So, yeah. To answer your question in the clearest sense, you might gain weight at OSUT, and it might not all be good.

Fortunately for me, I was able to lean out some at ABN school when I could start hitting the gym again on my own and had more control over my diet.