Stir flour, oats, pecans, the remaining 1/3 cup brown sugar, the remaining 1/2 teaspoon cinnamon and salt together in the medium bowl. Cut in butter with your fingers or a pastry blender until mixture is crumbly. Spread evenly over the apples. Bake until the topping is golden and the filling is bubbling, 45 to 55 minutes. Let cool for at least 20 minutes before serving.