In a large bowl, mix together the oats with chia seeds and PB2 peanut butter powder.
In another bowl, mash the banana with a fork until it's smooth. Add the ground cinnamon, vanilla extract, and optional sweetener, if using. Mix well.
Pour the banana mixture into the oats, then add the flaxseed milk. Stir everything together until well combined.
Divide the mixture into two serving containers (like mason jars or bowls). Cover and refrigerate overnight.
Enjoy as is, or top with a few banana slices or a sprinkle of cinnamon for extra flavor.