In a large skillet on medium heat, add in the olive oil and the Italian Sausage. Cook until partially done, stirring frequently. Add in the bell peppers and onions and cook until the sausage is completely cooked through, and the vegetables are soft. Set aside.
Pour the cooked pasta into a 9x13 baking dish, and add in the Italian Sausage mixture, marinara sauce, and 1 cup of the mozzarella cheese. Season with garlic powder and salt/pepper to taste and stir to combine. Top with the remaining cup of mozzarella cheese and bake for about 30 minutes.