Preheat oven to 425°F (220°C).

Grease a 12-cup muffin tin.

In a large bowl, combine oats, flour, sugar, baking powder, and salt.

In a separate bowl, mix milk, egg, and oil until combined.

Pour the wet ingredients into the dry ingredients and mix until just moistened. Do not overmix.

Gently fold in the raspberries.

In a small bowl, combine oats, brown sugar or white sugar, and cold, cut-up butter for the topping.

Crumble the topping mixture with your fingers until it resembles coarse crumbs.

Fill the prepared muffin cups 2/3 full with batter.

Sprinkle the topping evenly over the muffin cups.

Bake for 20 to 25 minutes, or until a toothpick inserted into the center comes out clean.

Let the muffins cool in the muffin tin for a few minutes before transferring them to a wire rack to cool completely.