Preheat oven to 350°. In a large bowl, beat the butter, brown sugar, cinnamon and salt until crumbly. Add flour and oats; mix well. Press 3 cups oat mixture into the bottom of a greased 13x9-in. baking dish., In another bowl, mix the first 5 filling ingredients. Add apples and vanilla; toss to combine. Spoon over oat layer. Dot with butter; sprinkle with remaining oat mixture., Bake, uncovered, until golden brown and apples are tender, 45-50 minutes. If desired, serve with toppings.