Preheat oven to 350 degrees F (175 degrees C). Line a muffin tin with liners.
Whisk buttermilk, bananas, and eggs together in a bowl; gradually stir in coconut oil.
Combine brownie mix, oats, all-purpose flour, cocoa powder, baking powder, and cinnamon in a large bowl. Make a well in the center and pour in buttermilk mixture. Stir together until batter is just moistened.
Divide batter among muffin liners, filling each 2/3 full.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 25 to 30 minutes. Cool in the pan, 10 to 15 minutes. Move to a rack to cool completely, about 20 minutes.
