Preheat oven to 425 F. Place 12 muffin cup liners in muffin tin. Lightly grease liners with non-stick cooking spray.
To make topping, combine oats, brown sugar, flour, and cinnamon in a bowl.
Stir in 1 tablespoon melted butter until combined. Reserve remaining melted butter.
Set topping aside.
In a large bowl, whisk Greek yogurt and milk together until combined.
Whisk in oil and reserved melted butter until combined.
Add vanilla and egg, and whisk in until combined.
Whisk in dry ingredients just until incorporated. Do not overmix.
Fold in blueberries.
Divide batter evenly among prepared muffin cups.
Sprinkle topping evenly over muffin batter, if using.
Bake at 425 F for 5 minutes.
Reduce oven temperature to 375 F; bake an additional 13 minutes, or until a wooden pick inserted in center comes out clean.
Cool muffins in pan for 5 minutes.
Remove from pan; cool completely on a wire rack.
Notes
If omitting the topping, I recommend using vanilla non-fat Greek yogurt for additional sweetness and flavor.
Anything from skim to whole milk will work.
I use finely ground whole-wheat flour. I've also used white whole-wheat flour. If you prefer, you can use all-purpose flour or half whole wheat and half all-purpose flour.
Nutrition values per muffin without topping: 199 kcal, 7 g fat, 29 g carbs, 5 g protein, 14 g sugar.