Pour the oat milk (or your choice of milk) into a two-cup liquid measure with high sides or a smoothie making container.
Slice the banana into the milk and add vanilla and maple syrup, if desired.
Use a handheld blender, smoothie tool, or regular blender to whip the oat milk mixture until smooth and uniform.
Vigorously stir in the chia seeds, distributing evenly throughout.
Cover and refrigerate for four hours or overnight. For individual servings, divide equally and pour into two small jars or bowls.
Serve topped with banana slices, berries and/or Greek yogurt.
Notes
The maple syrup is completely optional although if you want to serve this as dessert, you shouldn't leave it out. As a dessert, you might even want to use whipped topping with sliced bananas plus a drizzle of maple syrup on top!