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, salt, and ground cinnamon. Mix well.
In another bowl, whisk together the milk, maple syrup (or honey), egg, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and stir until just combined.
Gently fold in the diced peaches, chopped pecans, and raisins or dried cranberries if using.
Pour the mixture into the prepared baking dish and spread it evenly.
Bake in the preheated oven for 30-35 minutes, or until the top is golden brown and the oatmeal is set.
Allow to cool for a few minutes before serving. Enjoy warm or at room temperature.