Preheat oven to 375 degrees. In a shallow bowl or on a plate, combine cornmeal, salt, pepper, garlic powder, chili powder and oregano; firmly press chicken into mixture to coat completely.
Heat the butter and olive oil in a 9- x 13-inch pan in the oven until the butter has melted. Place chicken in pan and turn to coat; bake, uncovered, for 30 minutes then turn and cook for 20 to 30 minutes.
Meanwhile, prepare salsa: In a medium-sized serving dish, combine remaining ingredients (tomato through cilantro); serve over baked chicken.