Prep your pan and oven: Heat the oven to 350°F (175°C). Line a 12-cup muffin pan with paper liners or grease lightly.
Mix the dry ingredients: In a large bowl, whisk oats, whole-wheat flour, baking powder, baking soda, cinnamon, and salt until combined.
Mash the bananas: In a separate bowl, mash the bananas until mostly smooth. A few small lumps are fine.
Blend wet ingredients: To the bananas, add eggs, Greek yogurt, milk, maple syrup (or honey), vanilla, and oil.
Whisk until the mixture looks smooth and cohesive.
Combine wet and dry: Pour the wet mixture into the dry. Stir gently with a spatula just until you no longer see dry flour. Do not overmix or the cupcakes can turn dense.
Fill the cups: Divide the batter evenly among the liners, about 3/4 full.
Tap the pan lightly on the counter to settle.
Bake: Bake 16–20 minutes, until the tops are set and a toothpick comes out clean or with a few moist crumbs.
Cool completely: Let cupcakes rest in the pan 5 minutes, then move to a rack to cool. Cool fully before frosting.
Make the frosting: In a bowl, beat the Greek yogurt and cream cheese until smooth and thick. Add maple syrup and vanilla, then beat again.
Taste and adjust sweetness if needed. For extra firmness, chill the frosting 20–30 minutes.
Frost and serve: Spoon or pipe frosting onto cooled cupcakes. Add sliced banana, a dusting of cinnamon, or a few sprinkles if you like.