For streusel, mix ½ cup oats, 2 Tbsp brown sugar, and ½ tsp cinnamon, then cut in 2 Tbsp cold butter until crumbly.
In a bowl, mix 1½ cups oats, 1¼ cups flour, ½ cup brown sugar, 1 tsp baking powder, ¾ tsp baking soda, 1 tsp cinnamon, and ½ tsp salt.
Stir in 1 cup applesauce, ½ cup milk, 3 Tbsp oil, ½ tsp vanilla, and 1 egg until combined.
Fill muffin cups ¾ full.
Top with streusel.
Bake 15–18 minutes until done.
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.