Heat oven to 350°F. Spray bottom only of 13x9-inch pan with cooking spray.
In large bowl, mix brownie mix, oil, water and eggs until well blended. Pour batter in pan. Bake 22 to 25 minutes or until toothpick inserted 2 inches from side of pan comes out clean. Cool completely on cooling rack, about 1 hour.
In large bowl, stir softened ice cream and 2 cups of the crushed cookies. Spread evenly over brownies in pan. Freeze uncovered 20 minutes. Spread whipped topping evenly over ice cream layer. Cover and freeze at least 4 hours until firm.
Sprinkle remaining 2 cups crushed cookies over top. Cut into 5 rows by 4 rows. Store tightly covered in freezer.
To soften ice cream, remove from freezer to refrigerator 20 to 30 minutes before needed.
To easily crush Oreo™ cookies, place cookies in resealable gallon-size food-storage bag; seal, and pound with rolling pin until coarsely crushed.