Preheat the oven to 375°F (190°C). Lightly oil a 12-cup muffin pan or spray liners with oil.

In a large bowl, mash the ripe bananas until mostly smooth. Add the egg, Greek yogurt, honey, and vanilla, then whisk until creamy.

Add the oats, flour, cinnamon, baking powder, baking soda, and salt. Stir gently until just combined, then fold in the chopped walnuts.

Divide the batter evenly among the 12 cups using a spoon or small ice cream scoop.

Bake at 375°F (190°C) for 20–25 minutes, until the tops are golden and a toothpick inserted comes out clean.

Cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.