Heat oven to 350°F and line a baking sheet with parchment paper; set aside.
In a large bowl or stand mixer fitted with a paddle attachment, mix together the cake mix, brown sugar, butter, peanut butter, milk, egg, and vanilla on medium speed until fully combined.
Stir in the oats, M&Ms, and chocolate chips just until combined.
Use a medium cookie scoop, drop balls of dough 2 inches apart on the prepared baking sheet.
Add sprinkles to the tops of the cookie dough balls, gently pressing them into the dough.
Bake for 12 to 15 minutes. Immediately after coming out of the oven, gently press candy eyes into the cookies (don't add them before baking or they will melt).
Cool for 5 minutes on the cookie sheet before transferring to a cooling rack to cool completely.