In a large container, combine 1/4 cup balsamic vinegar, 1 tbsp dijon mustard, 2 cloves garlic, 1/2 tsp dried oregano, and 1/4 tsp each salt and ground black pepper.
Add 4 chicken breasts and marinate in the refrigerator at least 2 hours or overnight.
Place chicken on a baking sheet, discard marinade and bake for 400°F for 25 to 30 minutes, or until cooked through.