Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking dish.
In a large bowl, combine the rolled oats, diced apples, brown sugar, maple syrup, cinnamon, nutmeg, and salt. Mix well.
In another bowl, whisk together the milk, vanilla extract, and eggs until well combined.
Pour the wet ingredients into the dry ingredients and stir until everything is evenly mixed. If using, fold in the walnuts and raisins or cranberries.
Pour the mixture into the prepared baking dish and spread it out evenly.
Bake in the preheated oven for 35-40 minutes, or until the top is golden brown and the oatmeal is set.
Allow to cool for about 10 minutes before serving. Cut into squares and enjoy warm.