Combine rolled oats, chia seeds, and vanilla extract in a 16-ounce glass jar.
Add the mashed banana, Greek yogurt, and maple syrup to the dry ingredients.
Pour in the milk and stir vigorously until the mixture is homogeneous and no clumps remain.
Gently fold in half of the sliced strawberries to ensure even distribution.
Seal the container with an airtight lid and refrigerate for a minimum of 6 hours, or ideally overnight.
Remove from refrigeration, stir to redistribute moisture, and garnish with the remaining strawberry slices prior to consumption.