Preheat the oven to 350°F (177°C). Prepare two 8 inch round cake pans by spraying the inside of each pan with cooking spray. Cut round pieces of parchment paper out in the shape of the pans and place one into each pan. Place Oreos into the bottom of the pan in an even layer until you can’t fit anymore (16 Oreos per pan).
If making your own buttermilk (see notes), make it and set aside. In a medium bowl or large liquid measuring cup, whisk together oil, buttermilk, vanilla, vinegar, food coloring, and eggs, then set aside.
In the bowl of a standing or handheld mixer, whisk the flour, sugar, salt, baking soda, and cocoa powder together.
Add the butter into the bowl with the dry ingredients and use the paddle attachment on your mixer to mix until combined and crumbly like sand (photos in post above). Add in the wet ingredients in and mix until combined, scraping along the sides of the bowl as you go. Mix until there are no more lumps.
Divide the cake batter between the cake pans then bake for 22-32 minutes, or until a toothpick inserted into the middle of the cakes comes out clean. Let the cakes cool in the pan for 20 minutes then transfer to a cooling rack until completely cooled. Meanwhile, make the Oreo buttercream.