Preheat oven to 325 degrees F. Grease and flour a bundt pan very well.
In a large mixing bowl, whisk together the flour, oats, baking soda, baking powder, salt, cinnamon and nutmeg. Set aside.
In another bowl, add the whole eggs, egg yolk, vanilla extract, buttermilk, softened butter, vegetable oil, packed brown sugar and granulated sugar. Mix very well with a whisk. Don't worry if the butter clumps up a bit.
Pour the liquid ingredients into the dry ingredients and mix with an electric mixer on medium just until the ingredients are well incorporated. Be careful not to mix too long. Mixing for over a minute is too long and can produce a dense cake. Stop mixing once the ingredients are well combined. It's okay if there are a few small lumps.
Add the chopped apples, raisins and optional walnuts and mix by hand with a wooden spoon or silicone spatula.
Pour into the prepared bundt pan an bake for approximately 60-65 minutes. The cake is done when a toothpic inserted into the cake comes out with just a few moist crumbs on it or clean, but no raw batter.
Remove the cake from the oven and let sit on a wire rack, in the pan for about 40 minutes, then remove from the pan and let cool completely on the rack. Make sure to let this cake sit in the pan for around 40 minutes before removing it or it will stick and potentially fall apart.
For the glaze:
Once the cake has cooled to almost room temperature, make the glaze.
Melt the butter in a saucepan over medium heat.
When the butter is completely melted, turn the heat to low and immediately add the brown sugar, vanilla and buttermilk.
Stir until the brown sugar is completely dissolved.
Turn the heat off and add the confectioner's sugar and stir well. If it's clumpy, just keep stirring until most of the clumps are dissolved. The rest will dissolve as it sits for a bit or use a whisk to break up the clumps. If there are a few stray clumps, pour the mixture through a strainer.
Set the glaze aside at room temperature (not in the fridge) to cool for about 10 minutes until it thickens and firms up a bit. Make sure to stir it occasionally as it cools down. It will crystallize a bit, but that's okay. Stirring it occasionally helps with this.
Spoon the glaze over the cake. If the glaze thickens up too much, you can heat it for a short time in the microwave, but only for a few seconds.
Let the glaze set and firm up on the cake for about 10 minutes before cutting and serving.
*After baking the cake, make sure to leave the cake in the pan for around 40 minutes before trying to remove the cake from the pan. If you try to turn the cake out of the pan too soon, it will fall apart on you.This cake can be left at room temperature for about a day, but store any leftovers, well covered, in the fridge for up to 3-4 more days. This cake is best served at room temperature or warm. To serve leftovers, cut a slice and microwave it for about 10-15 seconds, or let it sit at room temperature for about 20 minutes.Make sure to check out the TIPS & FAQs for this recipe in the blog post, which may answer any questions you may have.*This recipe card may contain affiliate links. As an Amazon Associate, I earn from qualifying purchases.