01 - Preheat oven to 350°F. Grease a 9-inch square baking dish or line with parchment paper.
02 - In a large mixing bowl, combine rolled oats, all-purpose flour, packed brown sugar, baking powder, baking soda, salt, ground cinnamon, and ground nutmeg. Stir until evenly blended.
03 - In a separate bowl, whisk together eggs, milk, melted butter, and vanilla extract until thoroughly combined.
04 - Pour wet mixture into dry ingredients. Stir until batter is just combined.
05 - Gently fold in walnuts or pecans and raisins or dried cranberries, if using.
06 - Transfer batter into prepared baking dish, spreading evenly.
07 - Bake for 35 to 40 minutes or until top is golden brown and a toothpick inserted in the center comes out clean.
08 - Allow cake to cool in dish for at least 10 minutes before slicing and serving.