Preheat oven to 400F. Spray muffin liners with non-stick cooking spray.
Make streusel topping first: mix oats, brown sugar and cinnamon together. Once combined, cut in butter. I use 2 butter knives and literally "cut" the butter into the mixture. When you are done your streusel should resemble small crumbles.
For the muffins, mix together dry ingredients: oats through salt.
Slowly mix in applesauce, milk, oil, vanilla and egg. Stir until well combined.
Use a large baking scoop to fill prepared muffin cups 3/4 full.
Spring a generous amount of streusel on the top of each muffin.
Bake at 400F for 15 to 18 minutes or until cooked through.
Notes
This is another recipe that I'd recommend freezing. That way you can just pull out the amount you need. I thaw and heat them up in the microwave in just a minute.