Preheat the oven to 350 degrees F (180 degrees C). (If you are using a dark pan, preheat the oven to 325 degrees F (170 degrees C). Grease a 9x13-inch baking dish.
For cake, whisk eggs, oil, and water together in a large bowl. Add cake mix; mix until smooth. Fold in 1 cup crushed Oreos. Pour batter into the prepared pan and spread evenly.
Bake in the preheated oven until a toothpick inserted in the center comes out clean, 28 to 32 minutes. Let the cake cool completely, about 20 minutes.
For tres leches soak, whisk sweetened condensed milk, evaporated milk, whole milk, and optional vanilla together in a bowl until fully combined; stir in 1/2 cup crushed Oreos.
Use a fork or skewer to poke holes all over the surface of the cooled cake. Slowly pour milk mixture evenly over the cake, allowing it to soak in. Cover and refrigerate for at least 4 hours, or overnight for the best texture.
For topping, fold 1/2 cup crushed Oreos into Cool Whip. Spread evenly over chilled cake. Decorate with extra crushed Oreos and mini Oreos. Keep refrigerated until ready to serve. This cake is best served cold.
