Crush Oreos and mix with margarine; spread into a 9x13 pan and freeze. Soften ice cream and spread on top of the cookies. Refreeze. Bring to a boil the sugar, evaporated milk, vanilla, chocolate and margarine. Boil for 1 minute; then cool. Pour over ice cream and refreeze. Mix Cool Whip with nuts and spread over chocolate. Keep frozen until ready to serve.