Mash the bananas in a large mixing bowl with a fork or potato masher until mostly smooth, leaving a few small lumps for texture and a sweet aroma.
Pour in the melted butter or coconut oil and stir until the mixture is glossy and well combined.
Mix in the brown sugar and eggs, one at a time, whisking after each addition until the batter is smooth, fragrant, and slightly loosened.
In a separate bowl, whisk together the sifted flour, oats, baking powder, baking soda, cinnamon, and salt to ensure even distribution of dry ingredients.
Gently fold the dry mixture into the wet ingredients using a spatula or spoon, just until combined—being careful not to overmix for tender muffins.
If desired, fold in chocolate chips or chopped nuts for extra texture and flavor.
Spoon the batter evenly into the prepared muffin tin, filling each cup about three-quarters full to allow room for rising.
Bake in a preheated oven at 180°C (350°F) for 20-25 minutes, until the tops are golden brown and a toothpick inserted into the center comes out clean.
Remove the muffins from the oven and let them rest in the tin for about 5 minutes, allowing the crumb to set and aroma to intensify.
Transfer the muffins to a wire rack to cool completely, or enjoy them warm for a tender, moist bite with a hearty chew.
Serve and enjoy these comforting banana oat muffins as a quick breakfast or satisfying snack, with a spread of butter or a drizzle of honey if desired.