In a large mixing bowl, combine the rolled oats, milk, Greek yogurt, honey or maple syrup, vanilla extract, baking powder, cinnamon, and salt. Stir until well combined.
Gently fold in the blueberries and nuts, if using.
Pour the mixture into a greased 9x9-inch baking dish, spreading it evenly.
Cover the dish with plastic wrap or a lid and refrigerate overnight (or for at least 4 hours) to allow the oats to soak up the liquid.
Preheat your oven to 350°F (175°C).
Remove the dish from the refrigerator and bake for 30-35 minutes, or until the top is golden brown and set.
Allow to cool for a few minutes before slicing into squares and serving. Enjoy warm or at room temperature.