In a mason jar or an airtight container, combine the rolled oats, almond milk, chia seeds, flaxseeds, cinnamon, and a pinch of sea salt. Stir everything together until well combined.
Add in the mixed berries, layering them on top of the oat mixture. If you prefer, drizzle honey or maple syrup over the top for extra sweetness, though it's optional.
Cover the jar with a lid or seal the container, and place it in the fridge to chill overnight (at least 6 hours).
In the morning, stir the oats, add the optional Greek yogurt for extra protein, and enjoy your overnight oats for weight loss!