Preheat the oven to 350?F. Prepare three 6-inch or two 8-inch cake pans by spraying with baking spray and adding parchment paper to the bottom.

Grind the oatmeal into fine crumbs and mix with cake flour, baking powder, baking soda, cinnamon, and salt in a bowl. Whisk to combine and set aside.

In a mixer, cream the butter until creamy. Add vegetable oil, granulated sugar, and brown sugar; mix on medium-high for 2 minutes.

Add eggs one at a time while mixing on low speed. Mix in sour cream and vanilla, then beat on high for 1 minute.

Gradually add the dry ingredients on low speed, then add the milk slowly and mix until just combined; scrape the bowl and stir by hand if needed.

Divide the batter between the pans and bake for 30-35 minutes. Cool in the pan for 5 minutes before transferring to a wire rack to cool completely.

To make Marshmallow Creme Filling, whip the butter until smooth, then mix in powdered sugar until combined. Add marshmallow creme, vanilla, and salt; mix until uniform.

For Vanilla Buttercream Frosting, cream the butter until light in color, then gradually add powdered sugar, mixing until fully incorporated. Add vanilla, milk, and salt; mix until smooth.

Level and torte the cooled cake layers. Pipe a circle of buttercream on the first layer's edge, fill with half of the marshmallow filling, and repeat with the next layer.

Place the final layer upside down on top, crumb coat with buttercream, and refrigerate for 20 minutes.

Finish frosting the cake and decorate with crumbled Oatmeal Creme Pies. Pipe swirls on top and place quartered creme pies in between.