Preheat the oven to 400 F. Line a 12-cup muffin tin with paper liners.
Pour oats and buttermilk in a large bowl, allow to sit and absorb for 10 minutes if you have the time.
Add the oil, brown sugar, eggs and vanilla. Mix until combined.
Stir in the baking powder, baking soda, salt, cinnamon, nutmeg and flour until just combined. Fold in blackberries.
Divide the batter into the prepared muffin cups and top with sugar.
Bake for 16-20 minutes, or until a light golden brown rim has formed along the edges. Allow to cool slightly.
If your blackberries are very large you can cut them in half before tossing into the batter to help with portion size.If using frozen blackberries, do not allow to thaw first. Buttermilk can be subbed with regular milk if preferred.You could sub in gluten free flour to make these gluten free.