Soak oats in 3/4 cup of the buttermilk 10 minutes.
Meanwhile, whisk together flour, baking powder, baking soda, cinnamon, nutmeg, and salt in a large bowl.
Stir egg, butter, brown sugar, remaining 3/4 cup and the 2 Tbsp buttermilk and oat mixture into dry ingredients until just combined.
Heat a griddle over medium heat until hot and lightly spray with oil. Working in batches, pour 1/4 cup batter per pancake onto griddle and cook until bubbles appear on surface and undersides are golden-brown, about 1 minute. Flip with a spatula and cook other side, about 1 minute more.
Blueberry Sauce: Combine sugar and cornstarch in a saucepan; stir in water. Bring to a boil over medium heat; boil for 3 minutes, stirring constantly. Add berries. Reduce heat and simmer for 8-10 minutes, until berries burst. Stir in butter until melted. Serve warm.