For streusel topping: Stir together brown sugar, flour, and butter until crumbly. Stir in oats and pecans. Set aside.
For filling: Preheat oven to 375 degrees. Lightly grease an 11 by 7 inch dish. Whisk flour, sugar, and milk just until blended. Whisk in melted butter. Pour into prepared pan. Top with banana slices and streusel topping. Bake for 40-45 minutes or until brown and bubbly.