Add oats, milk, walnuts, chia seeds, flax seeds, vanilla, and maple syrup to a glass jar. Stir until well combined and place lid tightly onto jar. Refrigerate overnight.
To serve, remove from refrigerator and remove lid. Top with fresh berries and maple syrup. To serve warm, remove lid and place into a microwave for 30 second intervals until warmed throughout, about 1 1/2 to 2 minutes. Remove from microwave and top with fresh berries and maple syrup.
Time does not include resting time in refrigerator overnight. If you require gluten-free, use gluten-free oats in this recipe.