For the frosting:
1 cup butter, at room temperature
3 1/2 cups powdered sugar
1 teaspoon milk
Pinch of salt
1 teaspoon vanilla extract
Preheat oven to 350ºF. Line 2 muffin tins with cupcake wrappers, filling 16 of the wells.
Make the cupcakes: In a large mixing bowl, whisk together the eggs, oil, and water until smooth. Add the cake mix, and whisk until combined.
Fill each of the lined cupcake wells 1/3 of the way full with the batter. Add 1 Oreo into the middle of each well, and top with more batter until the well is 2/3 of the way full. Repeat until all the Oreos are gone.
Bake cupcakes for 10 to 15 minutes or until a toothpick inserted into the center of the cupcake comes out clean. Cool on a wire rack.
Make the frosting: In a stand mixer, cream together the room-temperature butter and powdered sugar until light, fluffy, and smooth. Add milk, a pinch of salt, and vanilla extract, and continue mixing for 3 to 4 minutes or until the frosting is completely mixed.
Place frosting into a piping bag, and pipe onto each cupcake. Garnish with sprinkles.