Preheat the oven to 350 F/180 C. Line three 8-inch cake pans with parchment paper on the bottoms and set them aside.
Bring a pot of water to a boil. Add in the oats, stir, and remove the pot from the heat. Cover the pot for 20 minutes. Let the oatmeal cool to room temperature before using.
Whisk together the flour, baking soda, salt, cinnamon, and nutmeg and set aside.
In a stand mixer, beat the butter and sugars together on medium speed for 5 minutes until light and fluffy. Add in the vanilla, eggs, and sour cream and mix until combined, scraping the sides of the bowl as necessary.
Mix in the cooked oats and mix until just combined. Then fold in the flour mixture, mixing until just combined. It will be a very thick batter!
Spread batter into pans (around 600g each). Bake for 20-22 mins, until a toothpick comes out covered in a few moist crumbs. Cool in pans on a wire rack, then run a spatula around the edges to flip them out.