Preheat oven to 400 degrees F. In a medium bowl combine the flour, oats, and brown sugar. Cut in butter until mixture resembles coarse crumbs. Stir in nuts. Pat mixture lightly into an ungreased 13x9x2-inch baking pan. Bake for 10 to 15 minutes. Remove from oven. While still warm, stir nut mixture to crumble. Cool.
Spread half of the crumbs in a 9x9x2-inch pan; drizzle about half of the ice cream topping over crumbs in pan. Place ice cream in a chilled medium bowl; stir to soften. Spoon softened ice cream carefully over topping-drizzled crumbs. Drizzle with remaining topping; sprinkle with the remaining crumbs. Cover and freeze at least 6 hours or until firm. Let stand at room temperature for 5 to 10 minutes before serving.