Add the cake mix to a large bowl.
In a separate medium bowl, mix oats and milk together. Let mixture sit for about 20 minutes so oats can soften.
While oats are soaking in milk, preheat oven to 375 degrees and line a muffin tin with papers.
Add eggs and yolk, oil, and vanilla extract (if using) to the milk and oat mixture and whisk to combine.
Pour the wet ingredients into the bowl with the cake mix and stir just until combined, taking care not to over-mix.
Spoon batter into the muffin cups, filling each about ¾ of the way full.
Bake for 15-18 minutes, until a toothpick inserted into the center of a muffin comes out clean.
Let muffins cool in the tin for a few minutes before moving to a metal rack to cool completely.