For the Crust

1. Mix Oreos and butter and press into 9×13 baking dish. Place in freezer or fridge.

For the filling:

1. Beat softened cream cheese until smooth, next gradually add remaining ingredients, except the whipped topping. Beat until smooth and creamy.
2. Fold in whipped cream.
3. Smooth filling on top of cookie crumb crust and refridgerate for one hour.
4. While the filling and crust is setting, cook the topping as follows.

For the topping.

1. Add the blackberries, sugar and 1/4 cup water to a saucepan or skillet. Bring to a boil over medium-high heat and cook until the juices thicken slightly — about 4 to 5 minutes.
2. In a separate small bowl, stir the cornstarch and 2 tablespoons water together until smooth.
3. Add the mixture to the berries.,
4. Boil the berry mixture another time for 1 to 2 minutes.
5. Remove from heat and let cool.
6. Gently spoon mixture on top of cheesecake and refrigerate for several hours before serving.