I can't say anything about Army OCS, but I know you have to be pretty diligent at military dining facilities, and be willing to splurge out of necessity. Food is cooked to provide the most food for the most people, so expect tons of fried and starchy foods, and commercial salad dressings. There will always be a salad bar if the main dish looks like it was scraped off the drill sergeant's boot. There is supposed to be two options, and sometimes the option is a fish dish or chicken. The option is usually the one none of the other guys and gals are taking, so you can have extras. Load up on the fish dishes and vegetables if you can, and eat a salad at every meal. Don't be afraid of the rice or a potato here and there if you are doing a lot of demanding training.
When you get out of training, the commissary is a great place to get what need. The Organic sections are growing, and inexpensive. You can find a lot of foreign vegetables not found in most chain supermarkets. Unfortunately, grass fed beef won't be much of an an option there, but the meat is very affordable and seafood isn't bad. Most places have their own butchers so you can ask for special cuts or soup bones and offal. Sometimes you can find cuts of offal in the case pre-cut. I shop there as much as I can, and it has saved my little family from living in a cardboard box. The savings will spoil you.
Best of luck! There won't be a shortage of "food" in the military, you just have to be more selective than most, and willing to sacrifice. The quality will not be held to your personal standard, but if you need to eat pick the best option.