Preheat oven to 350°. Make streusel: In a large bowl, combine oats, flour, brown sugar, cinnamon, and salt. With your fingers, squeeze and rub butter into oat mixture until butter is completely blended and coarse lumps form. Set aside.
Make filling: In a large bowl, combine all ingredients except apples. Add apples and toss to coat. Transfer to a shallow 2 1/2- to 3 1/2-qt. baking dish, such as a 9- by 13-in. one. Scatter streusel on top.
Bake until streusel is browned and apples are tender when pierced, 55 minutes to 1 1/4 hours (some apple varieties bake faster than others), tenting with foil if streusel starts to get dark before apples are done. Let cool on a rack at least 20 minutes.