Preheat oven to 350 degrees. Grease an 8x8 square baking pan (or other medium baking pan) with coconut oil or cooking spray and set aside.
In a large mixing bowl, mix together the oats, baking powder, cinnamon, and salt. Set aside.
In another mixing bowl, whisk together the mashed banana, almond milk, egg, and vanilla extract. Add the wet ingredients to the dry ingredients and mix until everything is combined.
Fold in the blueberries, strawberries and raspberries and pour the oat batter into the baking pan.
Bake for 35 to 40 minutes or until the top is set and lightly golden brown.
Remove from the oven and allow to cool for 10 to 15 minutes. Slice and serve warm, or cool completely, cover tightly, and refrigerate until ready to enjoy.