Preheat the oven to 375°F. Grease 12 standard or 40 mini muffin cups with oil or line with paper liners. Set aside.
Add the liquid ingredients to the dry ingredients, using a rubber spatula to gently stir them together until just combined. Add the cranberries, stirring with as few strokes as possible. If you stir the batter too much, it will result in tougher muffins (we want tough kids, not tough muffins!).
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, cinnamon, and salt. 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.