Speaking from two different angles here. One involved 2 years of Air Force Jr ROTC in High School, involving lots of marching and mostly standing in formations outside for long periods of time. The first rule we learned was to not lock your knees while standing in formation, especially when it's warm. That's the easiest way to find yourself on the ground in a faint or make yourself nauseous. So make sure you are not unconsciously locking up your knees while standing for longer periods or when the rooms are particularly warm, which seems to happen frequently in certain circumstances. Move your feet around a little, and bend your knees from time to time, and if that's the issue, it should help out a lot.
Speaking from an OR learning environment ~ Other advice regarding eating enough is right on the money, especially early in the day, and plenty of protein and carbs. Even peanut butter crackers and bananas and yogurt is better than Starbucks alone for breakfast. Your brain runs on glucose, and it's running on overtime while you're in such a high-stress learning-intense environment, so don't forget to feed it! I used to precept Surgical Tech students and RNs in the scrub role, and my first question after greetings each morning was "Have you had a good breakfast?" Those who had, did well. Others who hadn't, not as well. I started keeping peanut butter crackers or cheese crackers in my locker, and a package of those could make the difference the first morning. After that, a decent meal was prerequisite for coming in my room!
There are lots of things you will be exposed to in the OR that you will almost never see outside of it, so don't be surprised if sights, sounds or smells or even associations that you never considered in advance produce troubling effects. I never ever was squeamish about anything anywhere, including blood, but my very first day observing in an OR, I found myself sitting outside in an anteroom, woozy from the smell of the Bovie smoke. I talked myself back in there twice, and refused to give in to it. It takes time to develop any kind of comfort level with new things, so be kind to yourself, and don't let anyone else bully or hound you into it before you're ready. If it seems to take longer than you feel should be "normal" then address it with someone with whom you have confidence.