Preheat oven to 375°F (190°C). Line a muffin tin with paper liners or grease lightly.
In a large bowl, mash the bananas thoroughly. Whisk in the eggs, maple syrup, oil, vanilla, and Greek yogurt until smooth.
In a separate bowl, combine the rolled oats, whole wheat flour, all-purpose flour, baking soda, baking powder, salt, and cinnamon.
Gently fold the dry ingredients into the wet ingredients until just combined. Avoid overmixing to prevent a tough texture. Fold in chocolate chips or walnuts if using.
Divide the batter evenly among the muffin cups — about ¼ cup (60ml) per standard muffin, or 2 tablespoons (30ml) per mini muffin.
Bake standard muffins for 18–22 minutes, or mini muffins for 12–15 minutes, until a toothpick inserted in the center comes out clean or with a few moist crumbs.
Cool in the tin for 5 minutes, then transfer to a wire rack to cool completely before storing.