Preheat the oven to 375°F (190°C). Line a muffin tin with paper liners or lightly grease it.
In a large mixing bowl, combine the rolled oats, all-purpose flour, granulated sugar, baking powder, and salt. Stir until evenly mixed, creating a dry base for the batter.
1 1/2 cups rolled oats
In a separate bowl, whisk together the milk, vegetable oil, and egg until the mixture is smooth and slightly frothy. Ensure all wet ingredients are well combined.
1 1/2 cups rolled oats
Pour the wet mixture into the dry ingredients. Use a spatula to gently fold everything together until just combined; do not overmix, as this can cause dense muffins.
Carefully fold in the blueberries, distributing them evenly throughout the batter. The batter should be thick but scoopable.
1 1/2 cups rolled oats
Spoon the batter evenly into the prepared muffin tin, filling each cup about three-quarters full. Place the tin in the oven and bake for 20 minutes, or until the muffins are golden brown and a toothpick inserted into the center comes out clean.