Heat the oven. Preheat the oven to 350°F.
Make the filling. Core, peel, and cut the apples into 1/2-inch chunks. Combine the apples with the cranberries, orange zest, granulated sugar, brown sugar, cornstarch, and cinnamon in a large bowl and toss until well coated. Pour into a 7 x 11-inch baking dish.
Make the topping. In a medium bowl, mix together the oats, flour, brown sugar, cinnamon, vanilla, and kosher salt. Cut the butter into small pieces, then use a fork or pastry cutter to work it into the oat mixture until fully distributed and a crumbly dough forms. Scatter the topping evenly over the fruit and smooth it into an even layer with your fingers.
Bake. Bake for 45 to 50 minutes, until the top is evenly golden brown and the filling is bubbly. Cool for 30 minutes to 1 hour, then serve warm. Leftovers can sit covered on the counter for up to 1 day; after that, refrigerate.
