Preheat the oven to 350°F (175°C) and prepare three 8-inch cake pans.
In a bowl, whisk together the flour, baking powder, baking soda, and salt.
In a larger bowl, beat the butter, sugar, and oil until creamy. Then, mix in the sour cream and vanilla. Alternate adding the milk and the dry ingredients. Gently fold in the whipped egg whites and chopped Oreos. Bake the mixture for about 35 minutes.
For the mousse, melt the white chocolate with half of the heavy cream. Once melted, mix it with the cream cheese and remaining whipped cream, folding in the chopped Oreos. Chill this mixture.
For the frosting, beat the remaining butter and cream cheese until smooth. Add the powdered sugar, salt, and vanilla, then stir in the pulverized Oreos.
For the ganache, heat the remaining heavy cream and pour it over the chopped semi-sweet chocolate. Whisk until smooth.
Assemble the cake by layering the baked cake, mousse, and frosting. Chill the assembled cake in the refrigerator for about 30 minutes.
Before serving, decorate with any extra frosting and whole Oreo cookies.