In a small bowl, mix together the milk, yogurt, honey, flaxseed, vanilla extract, and salt until they are well combined.
Add the rolled oats and mashed banana to the bowl, and stir until the oats are well coated in the mixture.
Cover the bowl with plastic wrap or a lid, and put it in the refrigerator overnight (or for at least 6 hours).
In the morning, give the oats a good stir to combine everything and enjoy them as is or add additional toppings such as sliced banana, berries, nuts, or a drizzle of honey, if desired.
If you prefer warm oats, microwave them for 30-60 seconds or heat them on the stove until they are warmed through.
Serve and enjoy your delicious and nutritious Overnight Oats with Banana and Flaxseed!