Prep and Soak Oats: Preheat your oven to 375°F and line a twelve cup muffin tin with paper liners. In a small bowl, combine the rolled oats and buttermilk. Let sit for ten to fifteen minutes to soften the oats.
Mix Dry and Wet Ingredients: In a large bowl, whisk together the flour, sugar, baking powder, baking soda, cinnamon, and salt. In another bowl, whisk the egg, melted butter, vanilla, and the soaked oat mixture. Pour the wet ingredients into the dry ingredients and fold gently with a spatula until just combined.
Fold in Blueberries & Prep Topping: Toss the blueberries with one tablespoon of flour so they do not sink to the bottom. Gently fold them into the batter. In a tiny bowl, mix the streusel ingredients until crumbly.
Bake: Divide the batter evenly among the twelve muffin cups. Sprinkle the crunchy oat streusel over each one. Bake for eighteen to twenty minutes until golden brown and a toothpick comes out clean. Cool for five minutes, then move to a wire rack.