Prepare the pan: Line a 9x13-inch baking dish with parchment paper, leaving some overhang for easy lifting.
Crush the OREOs: Crush the OREOs in a zip-top bag or food processor. Set aside about one-third for topping.
Make the OREO topping: Fold two-thirds of the crushed OREOs into the thawed whipped topping until combined.
Add the first layer: Arrange half of the ice cream sandwiches in the bottom of the pan, trimming pieces as needed to fit.
Add chocolate sauce: Spread a thin layer of chocolate fudge sauce or syrup over the ice cream sandwiches.
Spread the topping: Add half of the OREO whipped topping and spread it evenly into the corners.
Repeat the layers: Add the remaining ice cream sandwiches, another thin layer of chocolate sauce, and the rest of the OREO whipped topping. Sprinkle reserved OREOs on top.
Freeze: Freeze for at least 4 hours, or overnight for cleaner slices. Cover once the top is firm.
Slice and serve: Let the cake sit for 5–15 minutes, then slice with a warm serrated knife and serve.
