Preheat the oven to 375°F. Grease 12 standard or 40 mini muffin cups with oil or line with paper liners. Set aside.
Divide the batter evenly among the muffin cups, filling them about 3/4 full.
To make the streusel topping, in a small bowl, stir together the oats, brown sugar, walnuts, and cinnamon. Break the butter into little bits and use your hands to work it into the oat/sugar mixture. Sprinkle it over the muffins, dividing it evenly.
Note: If you don’t have oat flour in your pantry, you can make it by pulverizing 1 scant cup of rolled or quick oats in a blender. It should yield 1 cup oat flour.