Prepare chocolate cake according to package directions and let it cool completely. Cut into small cubes.
Prepare chocolate pudding using the pudding mix and milk. Chill until set.
In a large trifle dish, layer half of the cake cubes, half of the pudding, a layer of crushed mint Oreos, and half of the whipped topping.
Repeat layers with remaining cake, pudding, Oreos, and whipped topping.
Garnish with mint candies or chocolate shavings before serving.
Chill for at least 1 hour before serving for best flavor.