Preheat oven to 350F and line a muffin tin with 12 liners.
Add oats to blender and blend for about 30 seconds until it becomes a mostly fine consistency. It does not need to be completely fine like flour.
Add remaining ingredients except walnuts and blend until combined. You may need to take a spatula and scoop the sides down, then blend again until the batter is uniform.
Add most of the walnuts and mix in with a spatula.
Scoop about 1/3 cup batter into each muffin cup. Distribute any remaining batter evenly. Sprinkle the remaining walnuts on top.
Bake for 20-25 minutes until muffins are slightly browned on top and a toothpick comes out clean.
Allow muffins to cool in the tin for 5-10 minutes, then move to a wire rack to finish cooling.
Video
Notes
These muffins should keep for 3-4 days. They freeze well if you don't plan on eating them all right away.