Preheat your oven to 325 degrees F. Spray three 8-inch round cake pans with nonstick cooking spray, line the bottoms with parchment paper and spray again. Set aside.
In a medium sized pot, bring the water to a boil. Add in 2 cups of the oats, stir and remove the pot from the heat. Cover the pot and allow the oats to soak for 20 minutes, until all the water is absorbed. Let the oatmeal cool completely before adding it to the cake batter.
In a medium sized mixing bowl, whisk together the flour, cinnamon, baking soda, and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugars together on medium speed for 2 minutes until light and fluffy. Add in the vanilla and the eggs and mix until combined, scraping the sides of the bowl as necessary. Continue to mix for an additional 2 minutes. The batter should be smooth and lighter in color than when you started mixing.
Turn the mixer to low and add in the cooled cooked oats and the flour mixture, mixing until just combined.
Toss in the remaining 1/2 cup dry oats and stir.
Spread the cake batter into the prepared pans, about 20 ounces of batter in each pan, and bake for 25 to 30 minutes or until the cake is set in the center and a toothpick comes out with just a few moist crumbs on it.
Allow the cake to cool in the pans for about 10 to 15 minutes and then invert onto wire racks to cool completely. Level the cakes, if needed.
If you're not using the cake layers right away, wrap each layer in plastic wrap. You can freeze them up to a week or up to a month if you also wrap them in foil.