Gather the ingredients.
Combine oats, brown sugar, flour, coconut and pecans in a bowl.
Add softened butter and mix together by squishing the butter into the mixture until it is evenly distributed. It will seem dry.
Put cake mix, eggs, sour cream and mashed bananas in another bowl.
Mix until smooth.
Grease and flour a tube pan.
Put a thin layer of cake mix on the bottom.
Generously spread a layer of the oat mixture on top of the batter.
Spread a second thin layer of batter over the oat layer.
Generously spread the oat mixture over the second layer of batter.
Finish with a third and final layer of batter and oat mixture. I pat the oat mixture down gently since this is the top of the cake.
Place in the middle of 350°F preheated oven and back for 50-60 minutes.
Remove from oven when a tooth pick inserted in the middle comes out clean.
Cool for 5 min. In the pan.
Carefully remove from pan and leave on a wire rack to finish cooling.
Put on plate, slice and enjoy.
See the nice layers.
Here is a better view if the layers.