Preheat the oven to 350 degrees F. Grease an 8 by 8 inch baking dish. In a large bowl, combine oats, baking powder, cinnamon, salt, and ½ cup walnuts.
Stir in maple syrup and melted coconut oil. In another bowl, combine milk, eggs, and vanilla. Stir the milk mixture into the oat mixture until well combined. Pour into the prepared pan.
Top with raspberries and blueberries. In a small bowl, combine the remaining ¼ cup of walnuts with the brown sugar and sprinkle on top of the berries. Bake for 40 minutes. Serve with Greek yogurt and additional berries, if desired.