In a sealable glass jar or container, combine the rolled oats, protein powder, and chia seeds.
Stir the dry ingredients until the protein powder is evenly distributed.
Add the milk, Greek yogurt, and vanilla extract to the container.
Stir vigorously until all ingredients are fully incorporated and no clumps of protein powder remain.
Seal the container with a lid and refrigerate for a minimum of 6 hours, or ideally overnight.
Remove from the refrigerator, stir once more, and top with fresh berries or nuts before serving.