
Preheat oven to 350 degrees and line a muffin tin with liners.
In a large bowl, mash bananas until mostly smooth. Whisk in avocado oil, brown sugar, granulated sugar, eggs, vanilla, and sour cream until fully combined.
In a medium bowl, whisk together flour, baking soda, salt, cinnamon, and nutmeg.
Add the dry ingredients to the wet ingredients and stir gently until just combined. Do not overmix.
In another bowl, make the streusel by combining brown sugar, flour, oats, and cinnamon. Cut in the cold butter until the mixture forms crumbles.
Fill muffin liners about two -thirds full. Sprinkle generously with streusel topping.
Bake for 18 to 22 minutes or until a toothpick comes out clean.
Let cool about 10 minutes before serving.