Preheat oven to 350 F. Use parchment paper, silicone, non-stick or greased cookie sheets.
Mix together cake mix, dry instant pudding, and oats. Add the vegetable oil, sour cream, water, and vanilla; stir together until mixed well. Stir in your preferred mix-in. Roll dough (or drop if too sticky) into balls, and place 2 inches apart on the cookie sheet.
Bake for 14 to 15 minutes. Allow cookies to cool on baking sheet for a few minutes before moving them to a wire rack to finish cooling.
Recipe by Three Different Directions at https://www.threedifferentdirections.com/egg-free-cake-mix-cookies-recipe.html