Preheat your oven to 375°F (190°C).
In a large mixing bowl, combine dry ingredients: flour, oats, baking powder, baking soda, cinnamon, nutmeg, and salt. Set aside for later.
In a separate medium bowl, cream together the butter, granulated sugar, and brown sugar using a hand mixer or stand mixer until light and fluffy—about 2 minutes.
Beat in the egg until fully incorporated, then mix in the yogurt until smooth.
Gradually combine the wet mixture with the dry ingredients, stirring gently with a spatula. The batter will be thick, and it's okay if a little flour remains unmixed.
Fold in the diced apples gently, taking care not to overmix the batter.
Prepare a muffin tin with 12 cups, either by lining them with paper liners or spraying with nonstick spray. Divide the batter evenly, filling each cup about 85% full.
For the oat streusel topping, mix together brown sugar, oats, flour, butter, and cinnamon in a small bowl. Sprinkle generously over each muffin.
Bake the muffins for 20 to 25 minutes, or until a toothpick inserted in the center comes out clean. Allow them to cool in the pan for about 5 minutes, then transfer to a wire rack to cool completely.