Place mayonnaise, mustard, oregano, garlic powder, salt and pepper in a small bowl and mix.
If the chicken breasts are thick, put each of them between two layers of plastic wrap, on a chopping board and pound them to a thickness of approximately 1/4 inch.
Spread half of the mayo-mustard mixture evenly on top of each.
Preheat oven to 400 degrees F. Grease the bottom of a baking dish lightly with cooking spray or vegetable oil. Lay the chicken breasts on the baking dish, sauce side down.
Spread the rest of mayo-mustard mixture on top.
Evenly sprinkle bread crumbs over top.
Place in the oven and bake until cooked through, about 40-45 minutes. Serve with any kind of salad.