Preheat oven to 375 degrees F. Spray muffin pan with non-stick oil spray or line with baking cups. Set aside.
Place old-fashioned oats in a blender and blend until fine and the texture of flour. Place oat flour in a large mixing bowl.
Add baking soda, baking powder, salt and pumpkin spice and whisk well.
Add pumpkin puree, vanilla, oil, milk and vinegar. Mix well.
Divide batter among muffin cups. You may end up with two cups empty.
Bake in 375 degrees F oven for 7 minutes. Do not open the oven but lower the temperature to 350 degrees F. Set timer for 14 minutes more. Remove muffins from oven and cool completely.
Baking powder should be made with baking soda and cream of tartar, and be free of starch. You can find DIY recipes online. Baking soda is gluten-free and if you are adding chocolate chips to the muffins, make sure they are gluten-free as well. For a homemade Pumpkin Pie Spice mix, check out my recipe: https://www.crunchycreamysweet.com/homemade-pumpkin-pie-spice/ .