Preheat your oven to 350°F (175°C) and prepare your muffin tin.
In a large bowl, mix together the dry ingredients.
In another bowl, whisk together the wet ingredients.
Pour the wet ingredients into the dry ingredients.
Spoon the batter into the prepared muffin tin.
Prepare the streusel topping by mixing together butter, brown sugar, oats, and cinnamon.
Sprinkle the streusel topping over the batter in the muffin tin.
Bake for 20-25 minutes or until a toothpick comes out clean.
Cool in the pan for 5-10 minutes before transferring to a wire rack.
Enjoy your fresh apple cinnamon streusel muffins warm.