Start thawing the ice cream just to the point that it's like soft serve. Place all the cookies into a strong freezer bag and pound them with a hammer or rolling pin until you have a bag of crumbs. (Using the pre-crumbled Oreos will not work...you need the white stuff too.) Set aside about 1/2 cup of the Oreo crumbs. Melt margarine, and mix well with crumbs. Press into a 9x13 cake pan to form a crust, and place in freezer to firm up.
Spoon thawed but not soupy ice cream onto the crust and spread evenly. Return to freezer and allow ice cream to refreeze.
Spoon whipped topping and spread evenly over ice cream. Sprinkle remainder of crushed cookies over whipped topping and drizzle with chocolate syrup. Refreeze. Thaw slightly when ready to serve and cut like a cake.