Preheat your oven to 350°F (175°C).
Butter a 9x14x2-inch oval baking dish.
Peel, core, and cut the apples into large wedges.
Combine the apples with the orange zest, orange juice, lemon zest, lemon juice, sugar, cinnamon, and nutmeg.
Pour the mixture into the baking dish.
In the bowl of an electric mixer fitted with the paddle attachment, combine the flour, brown sugar, salt, oatmeal, and cold butter.
Mix on low speed until the mixture is crumbly and the butter is pea-sized.
Spread the topping evenly over the apples in the baking dish.
Place the baking dish on a sheet pan to catch any drips.
Bake for 1 hour until the top is browned and the apples are bubbly.
Serve warm or at room temperature.