Preheat the oven to 425°F.
Line a 12-cavity muffin tin with paper liners.
To make the topping, stir the oats, flour, brown sugar, cinnamon, and salt together in a small bowl. Add the melted butter and stir until the ingredients are combined and the dry ingredients are saturated with butter. Place the bowl in the fridge to chill while you prepare the muffin batter.
To make the muffins, whisk the flour, sugar, brown sugar, baking powder, baking soda, cinnamon, and salt together in a large bowl.
In a small container such as a liquid measuring cup, stir together the milk, oil, vanilla, and apple cider vinegar.
Pour the milk mixture into the bowl with the flour mixture. Stir everything together just until combined.
Gently fold in the berries.
Divide the batter among the muffin tins, then sprinkle with the topping, gently pressing it into the batter so it adheres to the muffins.
Bake the muffins for 8 minutes at 425°F, then turn the oven temperature down to 350°F and bake them for another 14 to 16 minutes, or until they're domed on top, golden brown, and a toothpick inserted into the center of a muffin comes out clean.
Place the tin on a cooling rack. Let the muffins cool completely before removing and serving them.