In a large bowl, stir together oats and buttermilk. Allow to sit at room temperature for 30 minutes to 1 hour.
Preheat oven to 400°F and line 22 to 24 muffin cups with paper liners. In a medium bowl, whisk together flour, baking powder, baking soda, salt, cinnamon, and nutmeg; set aside.
After oats have finished soaking, stir brown sugar and eggs into the oatmeal/buttermilk mixture. Mix in melted butter and vanilla, and then blend in flour mixture and optional add-ins (if using) until just combined.
Evenly divide batter between prepared muffin cups. Bake for 15 to 20 minutes or until tops of muffins are golden brown and toothpick inserted into center of muffin comes out clean. Cool muffins in pan for a few minutes before turning out onto a cooling rack to finish cooling completely.
If you don't have any buttermilk on hand, you can make your own by pouring 2 tablespoons of lemon juice (or white vinegar) into a 2-cup (or larger) liquid measuring cup. Fill the measuring cup up to the 2-cup line with milk (whole milk is preferred but low-fat/skim milk will work). Allow to sit for 5 to 10 minutes and stir before proceeding with recipe.