In a saucepan, cook pasta according to package directions. Drain.
Meanwhile, in a 12-inch skillet, cook sausage, onion, green pepper, and garlic until meat is brown and onion is tender. Drain off fat.
Stir the undrained tomatoes, tomato paste, water, oregano, and pepper into the mixture in skillet. Stir in the cooked pasta.
Spoon half of the mostaccioli mixture into a 3-quart casserole. Sprinkle with half of the cheese. Top with the remaining mostaccioli mixture.
Bake in a 350 degree F oven for 35 minutes. Sprinkle with the remaining cheese. Bake about 5 minutes more or until cheese melts. Makes 8 servings.