Heat olive oil in a skillet over medium heat; cook and stir Italian sausage until browned and crumbly, about 10 minutes. Use a slotted spoon to transfer sausage to prepared casserole dish. Drain drippings and reserve 2 tablespoons drippings in the skillet.
Melt 2 tablespoons margarine in a separate skillet over medium heat; cook and stir apple and onion slices until tender, about 8 minutes. Place apple and onion in casserole dish with sausage.
Lightly stir sausage, apple, and onion together; season with salt, black pepper, and oregano.
Brown flour in reserved drippings in skillet over medium heat. Whisk water into flour until thick gravy forms, about 3 minutes.
Pour gravy over sausage mixture in the casserole dish; spread mashed potatoes over the top with a spoon, creating peaks. Lightly brush mashed potatoes with 1 tablespoon melted margarine.
Bake in the preheated oven until the potatoes are lightly browned and the sausage mixture is bubbling, about 30 minutes.
