Position oven rack to the middle and pre-heat to 350 F.
Line a 12-cup muffin pan with paper or silicone liners. Set aside.
To a large bowl, add flour, oats, baking powder, baking soda, cinnamon, and salt. Whisk and set aside.
To a separate large bowl, add melted butter and brown sugar. Whisk until smooth and well combined. Add eggs, vanilla extract, and milk, and whisk until combined.
Add dry ingredients and chopped apples to wet ingredients. With a large wooden spoon or spatula, mix until just combined. Be careful not to over-mix.
Scoop batter into muffin liners, distributing evenly.
Transfer to the oven and bake for 18-25 minutes, or until tops are set and a toothpick placed in the center comes out mostly clean. Be careful not to over-bake these or they could dry out.
Let muffins cool in the pan for 15-minutes before transferring to a wire rack. Cool to room temperature before eating.
Calories: 180kcal, Carbohydrates: 26g, Protein: 4g, Fat: 7g, Saturated Fat: 4g, Polyunsaturated Fat: 1g, Monounsaturated Fat: 2g, Trans Fat: 0.2g, Cholesterol: 44mg, Sodium: 240mg, Potassium: 125mg, Fiber: 3g, Sugar: 11g, Vitamin A: 239IU, Vitamin C: 0.5mg, Calcium: 57mg, Iron: 1mg