Step 1: Mix the dry ingredients - In a mixing bowl, combine the rolled oats and chia seeds.
Step 2: Mix the wet ingredients - In another mixing bowl, whisk together the milk, Greek yogurt, maple syrup, and vanilla extract.
Step 3: Combine the wet and dry ingredients - Pour the wet ingredients over the dry ingredients, and mix until well combined.
Step 4: Add the frozen mixed berries - Add the frozen mixed berries to the mixture and stir gently to combine.
Step 5: Chill overnight - Transfer the mixture to a mason jar or an airtight container and refrigerate overnight, or for at least 6 hours.
Step 6: Top and serve - When ready to serve, give the mixture a stir and top with additional berries or coconut flakes, if desired.