Preheat oven to 400°. Rub a 13"-by-9" glass baking dish with oil. On a baking sheet, toss potatoes, 2 tablespoons oil, and 1/2 teaspoon salt.
Roast potatoes until lightly browned and cooked through, 15 to 20 minutes. Let cool slightly. Reduce oven temperature to 350°.
Meanwhile, in a large skillet over medium heat, heat remaining 2 tablespoons oil. Add onion and 1/2 teaspoon salt and cook, stirring occasionally, until slightly softened, 3 to 4 minutes. Add bell pepper, garlic, chili powder, cumin, and oregano and cook, stirring occasionally, until pepper is just slightly softened and spices are fragrant, about 2 minutes more. Stir in beans, tomatoes, tomato sauce, corn, and remaining 1/2 teaspoon salt and cook, stirring occasionally, until heated through, 1 to 2 minutes.
Pour half of chili into prepared dish. Sprinkle with half of potatoes. Top with remaining chili and potatoes.
Cover with foil and bake casserole until bubbling around the edges and hot in the center, 20 to 25 minutes. Uncover, top with cheese, and continue to bake until cheese is melted, 4 to 6 minutes more.
Sprinkle with scallions and serve with sour cream alongside.