Preheat the oven to 375°F. Line a 12-cup muffin pan with liners, and set it aside.
Toss the blueberries in a tablespoon of flour.
Mix the rolled oats, flour, baking powder, baking soda, salt, ground cinnamon, and light brown sugar in a large bowl.
Combine the egg, vanilla extract, and oil in a separate bowl.
Add the egg mixture and buttermilk to the dry mixture. Stir just until combined.
Stir in the mashed bananas and blueberries.
Divide the muffin batter among the paper-lined cups, filling each about ¾ of the way up.
To make the streusel, whisk the oats, flour, brown sugar, ground cinnamon, and salt. Work in the butter until coarse crumbs form. Sprinkle the streusel over each muffin.
Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.