Preheat your oven to 350°F (175°C) and grease a 9x9-inch baking dish.
In a large bowl, combine rolled oats, baking powder, ground cinnamon, and salt. Mix well and set aside.
In another bowl, whisk together milk, maple syrup, eggs, vanilla extract, and Greek yogurt until smooth. Pour this into the dry ingredients and stir until combined. Fold in walnuts or raisins if using.
Transfer the mixture to the prepared baking dish and spread evenly. Mix brown sugar and ground cinnamon for the topping and sprinkle over the top.
Bake for 30-35 minutes until golden brown and a toothpick inserted in the center comes out clean. Allow to cool for 10 minutes before slicing.