Preheat oven to 425F. Prepare a muffin pan with parchment paper liners or grease with a bit of butter.
In the bowl of a stand mixer (or using a handheld mixer) whisk together oil, egg and sugars on medium speed until pale and fluffy, about 3 minutes. Add in orange zest and sour cream, whisking 1 minute more.
In a large bowl, whisk together flour, oats, baking soda, salt and cinnamon. Add to the wet mixture and fold gently until all dry parts are incorporated, being careful not to overmix. Fold in cranberries and walnuts. Fill muffin liners to just below the top— a heaping 3 tablespoon size cookie scoop is the perfect measure.
Add streusel ingredients to a small bowl. Use a fork to blend until moistened and clumpy. Sprinkle evenly over the muffins. Bake about 5 minutes—reduce heat to 350F and bake an additional 12-15 minutes, until a toothpick inserted into the center of the muffin comes out clean. Cool muffins on a wire rack and store airtight about 3 days. Muffins can be individually wrapped and frozen for a few months. Let thaw at room temperature about an hour before warming and serving.