Prepare streusel topping: In a bowl combine butter, sugar and flour and crumble until you get coarse breadcrumbs. You can use your fingers or a fork. Add rolled oats and pecans. Keep refrigerated until you need to use it.
Preheat oven to 425 F (220 C). Line a muffin tin with paper liners or spray with cooking spray. I use Misto.
Beat eggs, sugar and oil until well blended.
Combine yogurt and baking soda and add to mixture. Mix until combined.
In a bowl combine dry ingredients (1 1/4 cup flour, 3/4 cup oat flour, 1/2 tsp salt, 1/2 tsp cinnamon and 1 1/2 tsp baking powder) and add to wet ingredients. Mix until incorporated.
Fold apples and pecans and mix until just combined.
Transfer muffin batter to muffin tin, filling each cup almost full.
Add about 1 tbsp of oat-pecan streusel topping to each muffin.
Bake on 425 F (220 C) for 5 minutes, then reduce temperature to 350 F and bake for 25-30 more minutes, until tips are golden and toothpick inserted comes out clean. (Apple baked goods may be tricky to bake, so please watch how your muffins bake and allow a few extra minutes. Baking time may depend on the kind of apples used, flour, moisture of the ingredients, oven and climate).
Take out of the oven, but let muffins cool in the pan for at least 10 minutes, then transfer to a cooling rack until they cool completely.
Keep muffins refrigerated in an airtight container for up to 7 days.