In a small bowl add the chia seeds and rolled oats to the milk. Mix to combine and allow to rest for at least 10 minutes, or, if planning ahead, you may do this overnight. Simply place the chia seeds, oats, and milk in a jar with a lid and refrigerate until ready to use.
When ready to make the smoothie, place the frozen strawberries and banana in the bowl of a high-powered, high-speed blender. Pulse, or blend on low just until the fruit is broken into tiny, crumbly pieces (if there are a couple larger chunks, that's ok).
Add the chia seed, oat, and milk mixture to the crumbly fruit and use a rubber spatula to carefully push the fruit from the sides and roughly mix to the center of the bowl. Pulse, or blend on low, once more.
Add the Greek yogurt to the blender or food processor and use the same rubber spatula to carefully push any fruit stuck on the sides of the bowl down to the center. Blend on LOW, pushing down the fruit from the sides if needed, until a thick and creamy smoothie is formed.
If needed, add additional Greek yogurt or milk to the smoothie, 1-2 tablespoons at a time.
Immediately remove smoothie from the blender or food processor into a bowl and top with desired fruit, nuts, or seeds.