Spray the inside of a 10-1/2 by 7 inch baking dish with cooking spray and place on a baking sheet.
In a large bowl, mix together the oats, sugar, baking powder, cinnamon, salt, half the walnuts/pecans, half the berries and half the chocolate chips. (Save the other half of berries, walnuts/pecans and chocolate chips for the top of the oatmeal).
In another bowl, whisk together the milk, egg, butter and vanilla extract. Add the oat mixture to prepared baking dish.
Arrange the remaining berries, walnuts/pecans and chocolate on top.
Add the banana slices to the top then pour the milk mixture over everything.
Gently shake the baking dish to help the milk mixture go throughout the oats.
Bake 35 to 40 minutes or until the top is nicely golden brown and the milk mixture has set.