In a medium bowl combine all-purpose flour, oats, baking powder and sugar. In another bowl, mix the softened butter and egg with an electric mixer, after about 2 minutes, add yogurt. Add the flour mixture and combine well using a spatula. Gently fold in 120g blueberries, using a spatula.
oat streusel topping
To make the oat streusel topping, combine oats, sugar and all-purpose flour in a small bowl. Using your fingertips lightly rub the butter into the mixture, to get a nice crumb. Set aside until needed.
bake and serve
Line the muffin pan with the muffin papers. Spoon the batter evenly into 2/3 of the muffin papers. Sprinkle remaining blueberries and oat streusel topping on top of each muffin. Place the pan in the oven and bake for about 25 minutes or until the topping is golden brown and the skewer inserted in the center comes out clean. Set the pan on a wire rack an cool completely. Serve.