Preheat oven to 400°F. Line your muffin tin with muffin cups.
In one mixing bowl, combine flour, oats, brown sugar, baking powder, and salt.
In a second bowl, mix together the buttermilk, oil, egg, and vanilla extract.
Mix the wet mixture into the dry mixture. Mix until about half combined, but not fully mixed.
Once the mixture is half combined, add in the blueberries and fold them in. Fold until the batter is fully mixed, but be careful not to over-mix; your muffins will end up extra dense if you do.
Fill each muffin cup about ½ to ¾ full, depending on how large you want your muffins and how many you want.
Make the topping by mixing the flour, oats, sugar, and cinnamon in a bowl.
Add the chilled butter and mix with a pastry blender until crumbly, like coarse sand with oats in it.
Add the topping to the filled muffins.
Bake for 15-18 minutes. Let cool slightly and eat while warm.