Preheat your oven to 350°F (175°C). Lightly grease three 6-inch cake pans with cooking spray or a thin layer of oil. Set aside.
Place 2½ cups of the rolled oats into a high-speed blender. Add the cinnamon, baking soda, salt, orange juice, applesauce, maple syrup, eggs, oil, and vanilla extract. Blend until the mixture becomes smooth and thick.
Pour the blended batter into a mixing bowl. Gently stir in the remaining 1½ cups of oats, shredded carrots, and raisins. Mix until everything is evenly combined.
Divide the batter evenly between the prepared pans. Bake for 20–25 minutes, or until the tops look lightly golden and a toothpick inserted in the center comes out clean.
Let the cakes rest in the pans for about 10 minutes. Carefully remove them and place on a wire rack to cool completely.
Once the layers are fully cooled, spread the healthy cream cheese frosting on top. Sprinkle with chopped walnuts if desired. Slice and serve.