Make cake: Adjust oven rack to middle position and preheat oven to 350°. Line bottom of a 9-inch cake pan with parchment paper and lightly coat the inside with non-stick pan spray. Sift flour, cocoa, baking soda, and salt into large bowl; set aside. In medium bowl, whisk sugar, sour cream, oil, eggs, and vanilla until smooth.
Whisk wet ingredients into dry ingredients until smooth. Pour batter into pan and bake until cake is just firm and toothpick inserted into center comes out with moist crumbs, 20 to 25 minutes. Let cake cool in pan for 15 minutes, then remove from pan to completely cool on wire rack, about 1 hour.
Carefully cut 6 Oreo cookies in half; set aside. Chop remaining cookies into 1/4s; set aside.
To Assemble Cake: Whip 2 cups of cream on medium-high speed to soft peak, spoon into large bowl and refrigerate. In same mixer bowl, whip remaining 2 1/2 cups cream, sugar, and vanilla to soft peak. Fold into already whipped cream.
Place about 1 cup whipped cream in bowl and refrigerate until ready to decorate cake. Fold chopped Oreos into remaining whipped cream.
Slice cake in half horizontally to create 2 layers. Place bottom layer on serving plate. Spread about 1/3 of Oreo whipped cream onto cake. Top with second cake layer and use remaining Oreo whipped cream to frost top and sides of cake. Chill in refrigerator for about 2 hours to allow cookies to soften.
Place reserved whipped cream in pastry bag fitted with star tip (re-whisk if necessary). Pipe 12 whipped cream rosettes around perimeter of cake and garnish with reserved Oreo cookie halves. Serve.