**Preheat Oven:** Preheat your oven to 375°F (190°C). Grease an 8x8-inch baking dish and set aside.
**Mix Dry Ingredients:** In a large bowl, combine the rolled oats, baking powder, cinnamon, nutmeg, and salt.
**Prepare Wet Ingredients:** In a separate bowl, whisk together the milk, maple syrup, egg, melted butter, and vanilla extract.
**Combine Mixtures:** Pour the wet ingredients into the bowl with the dry ingredients and mix until well combined. Fold in the diced apples, nuts, and raisins, if using.
**Bake:** Pour the mixture into the prepared baking dish and spread it out evenly. Bake for 35–40 minutes, or until the top is golden and the mixture is set.
**Cool and Serve:** Let the oatmeal bake cool slightly before serving. Enjoy warm or at room temperature.