Preheat oven to 400 degrees F and line regular muffin tin with liners or grease well.
Make the streusel in a small bowl by whisking together flour, sugar, oats and cinnamon. Add melted butter and stir with a fork until crumbly. Set aside.
Mix muffins in a large bowl by stirring together flour, baking powder, and salt. Set aside.
In a medium bowl, mix egg and sugar until combined. Add in yogurt, oil and vanilla.
Fold wet mixture into dry ingredients and mix together by hand.
Spoon batter into prepared muffin tin.
Cover the batter with the apples and the add streusel topping.
Place in the oven and reduce heat to 375 degrees F. Bake 18 to 20 minutes or until toothpick comes out clean in the center of a muffin.
Cool for 5 minutes in the pan then remove to a wire rack to cool completely.