Preheat the oven to 350 degrees F (175 degrees C). Bring a saucepan of water to a boil. Add the seashell pasta, and cook until tender, about 7 minutes. Drain. Meanwhile, heat the oil in a skillet over medium heat. Add the pepper and onion; cook and stir until tender. Stir in the cheese soup, salt and mustard. Mix in the macaroni and oysters so they are evenly distributed. Spoon half of the mixture into a 2 quart casserole dish. Cover with half of the Cheddar cheese. Top with remaining oyster mixture and remaining cheese. Bake covered for 15 minutes in the preheated oven. Then uncover, and continue baking for 20 minutes, or until hot and bubbly.