Preheat your oven to 180°C (350°F). Lightly grease or line a muffin tin with paper liners.
In a large mixing bowl, whisk together the whole wheat flour, rolled oats, baking powder, and salt until evenly combined. This creates a balanced dry base for your muffins.
In a separate bowl, stir together the brown sugar and melted butter or oil. Whisk in the eggs one at a time until the mixture is smooth and slightly frothy, about 30 seconds.
Add the leftover cooked oats to the wet mixture and fold gently with a spatula, ensuring they are evenly distributed. This step infuses the batter with a chewy, rustic texture.
Pour the wet mixture into the dry ingredients and fold gently until just combined. Be careful not to overmix; a few lumps are fine.
Gently fold in the blueberries, taking care not to crush them, so the juice stays vibrant and the berries remain whole.
Spoon the batter evenly into the prepared muffin tin, filling each cup about three-quarters full. Dot a few extra blueberries on top of each muffin for extra burst of flavor.
Bake the muffins in the preheated oven for 20–22 minutes, or until the tops are golden brown and a toothpick inserted into the center comes out clean.
Remove the muffins from the oven and let them cool in the tin for 5 minutes. Then transfer them to a cooling rack to cool completely or enjoy warm straight away.
Enjoy these rustic, chewy blueberry oat muffins for breakfast or a cozy snack, with their craggy tops and bursting berries making every bite satisfying.