In a large mixing bowl, stir together the 1 1/2 cups of rolled oats, buttermilk, melted butter, and oil. Set aside at room temperature for 20 minutes. The oats will absorb the liquid, turning into a thick and creamy base.
While the oats soak, preheat your oven to 425°F (218°C). Line a standard 12-cup muffin tin with paper liners, or spray the cups generously with a floured baking spray.
In a separate medium bowl, thoroughly whisk together the all-purpose flour, baking powder, baking soda, cinnamon, nutmeg, and sea salt.
Once the oats have finished soaking, whisk the dark brown sugar, eggs, and vanilla extract directly into the wet oat mixture until completely smooth.
Pour the dry flour mixture into the wet oat mixture. Using a rubber spatula, gently fold the ingredients together just until the flour streaks disappear (about 10-12 folds). Do not overmix, or the muffins will develop a tough texture.
Divide the batter evenly among the 12 muffin cups. They should be filled almost to the top (about 7/8 full) this is necessary for high domes. Sprinkle the tops evenly with the extra pinch of rolled oats and a generous dust of coarse sparkling sugar.
Place the tin on the middle rack and bake at 425°F (218°C) for exactly 5 minutes.
Without opening the oven door, reduce the temperature to 350°F (177°C) and bake for an additional 13 to 15 minutes.
They are done when the centers bounce back gently when pressed, or a toothpick inserted into the center comes out clean with a few moist crumbs attached.
