For streusel, mix oats, brown sugar, and cinnamon, then cut in cold butter until crumbly.
In a bowl, mix oats, flour, brown sugar, baking powder, baking soda, cinnamon, and salt.
Stir in applesauce, milk, oil, vanilla, and 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.