Preheat oven to 350°F (180°C). Line an 8x5-inch loaf pan with parchment paper. Set aside.
Prepare the flax egg by whisking the flaxseed meal and lukewarm water. Set aside for 10 minutes until a gel-like texture forms.
Peel the 3 ripe bananas, mash them with a potato masher or a fork and measure 1 cup + 1/4 cup of mashed bananas.
In a large mixing bowl, add mashed bananas, flax egg, melted coconut oil, vanilla, and light brown sugar. Stir to combine.
Stir in old-fashioned rolled oats, cinnamon, baking powder, baking soda, and oat flour.
Stir in chopped walnuts if desired and combine in the batter with a spoon.
Pour the banana bread batter into the prepared loaf tin.
Bake at 350°C (180°C) for 55-65 minutes or until golden brown and a skewer inserted in the center of the bread comes out clean or with few crumbs. You can tent the loaf pan after 30 minutes with a piece of foil to avoid the top to brown too fast.
Cool in the tin for 10 minutes, then lift out the bread from the pan using the hanging pieces of parchment paper
Cool the banana bread on a rack for 3 hours or until it reaches room temperature before slicing.
Slice the bread into 12 slices once it has cooled to room temperature. Store in the fridge in a sealed box for up to 4 days or freeze for later.
Coconut sugar can be replaced with cane sugar or brown sugar.