Make the crust: Combine the crushed oatmeal cookies, melted butter, brown sugar, cinnamon, and salt in a bowl until evenly moistened.
Press into the pan: Transfer the mixture to a 9-inch pie dish. Firmly press it across the bottom and slightly up the sides.
Chill the crust: Place the crust in the freezer for 15 to 20 minutes while preparing the filling.
Soften the ice cream: Let the vanilla ice cream sit at room temperature until soft enough to stir, but do not allow it to become completely melted.
Prepare the filling: Combine the softened ice cream, heavy cream, brown sugar, vanilla, cinnamon, and salt. Mix gently until smooth and creamy
Add the oatmeal cream pies: Fold the chopped oatmeal cream pie pieces into the ice cream mixture, reserving a few pieces for the topping.
Fill the crust: Spoon the mixture into the chilled crust and smooth the surface with a spatula.
Decorate: Sprinkle the reserved oatmeal cream pie pieces and crushed oatmeal cookies over the top. Add a light dusting of cinnamon if desired.
Freeze: Cover the pie tightly and freeze for at least 6 hours, preferably overnight
Slice and serve: Let the pie stand at room temperature for 5 to 10 minutes. Slice with a sharp, warmed knife and serve with whipped cream if desired.
