Preheat your oven to 350°F (175°C) and grease a 9x9-inch baking dish.
In a large bowl, combine the rolled oats, baking powder, cinnamon, and salt. Mix well.
In another bowl, whisk together the milk, maple syrup (or honey), eggs, melted coconut oil (or butter), and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and stir until combined.
Fold in the diced apples, nuts (if using), and raisins or cranberries (if using).
Pour the mixture into the prepared baking dish and spread it evenly.
Bake for 30-35 minutes, or until the top is golden brown and the oatmeal is set.
Allow to cool for a few minutes before slicing and serving.