Preheat the oven to 375°F (190°C) and line a muffin tin with paper liners or lightly grease it.
In a large mixing bowl, combine the rolled oats, all-purpose flour, brown sugar, cinnamon, baking powder, and salt. Whisk these dry ingredients together until evenly distributed.
In another bowl, whisk together the milk, vegetable oil, and eggs until smooth and slightly frothy.
Pour the wet mixture into the bowl with the dry ingredients, then fold gently with a spatula until just combined. The batter should be slightly lumpy but well mixed.
Carefully fold in the blackberries, distributing them evenly throughout the batter. Be gentle to prevent the berries from breaking too much.
Spoon the batter into the prepared muffin tin, filling each cup about three-quarters full. You can use a scoop or two spoons for neatness.
Optional: Lightly sprinkle additional cinnamon or sugar over the tops for extra flavor and a slight crunchy finish.
Bake in the preheated oven for 18–20 minutes, or until the tops are golden brown and a toothpick inserted into the center comes out clean.
Once baked, remove the muffins from the oven and let them cool in the tin for about 5 minutes, then transfer to a wire rack to cool completely.
Enjoy these warm or at room temperature, with a pat of butter or simply on their own. The muffins will keep in an airtight container for a few days.
