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 blended and coarse lumps form. Set aside.
Make filling: In a large bowl, combine all ingredients except apples. Add 2 tbsp. water and the apples and toss to coat. Transfer to a shallow 3- to 3-1/2-qt. baking dish, such as 9- by 13-in. dish. Scatter streusel on top.
Bake until streusel is browned and apples are tender when pierced, 55 minutes to 1 1/4 hours, tenting with foil if streusel starts to get dark before apples are done. Let cool on a rack at least 20 minutes, then serve warm or at room temperature.