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.

My opinion, as both a personal trainer and a career Soldier, is don’t worry about it. You’ve already done the right thing by preparing yourself physically. As an 11B you will see and feel changes in your body as a result from that type training regimen. Men always want to lift weights as a means of getting “big”, because they see that as a sign of manliness and strength. I’ve met several Soldiers that did nothing but body weight training and were in amazing shape and looked “jacked”. One of them was a former SFOD-D officer. They also benefited from being more injury free and, most importantly, able to do their Army job more effectively. Hell, if worrying about your appearance was so important, no one would attend Ranger School, SERE School, BUD/S, etc.

If size means that much to you, once you’re done with OSUT, you can go back to the weight room and put it back on in no time.