Combine ground beef, bread crumbs, onion, milk, ketchup, eggs, salt, pepper, and oregano. Mix gently until well blended. Shape into a loaf (about 8x5 inches) on a 13x9x2-inch baking pan. Arrange tomato slices on top of the loaf. Bake 1 hour and 15 minutes at 350°. Place cheese slices, overlapping, over the tomatoes the last 5 to 10 minutes.
Calories per serving (based on 6 servings): 276