Preheat oven to 350°F (180°C). Spray cupcake/muffin pan with nonstick cooking spray then set aside.
Whisk the flour, cocoa powder, and salt together in a medium bowl. Set aside.
Whisk the melted butter, vegetable oil, sugars, vanilla, and eggs together until well combined. Sift in the dry ingredients and fold until just combined; be careful not to overmix. Batter will be thick.
Equally spoon the batter into each cavity in the prepared cupcake/muffin pans and drop an Oreo in each cavity, smoothing the top out and ensuring the Oreo is completely covered.
Bake for 20-22 minutes. To test, insert a toothpick into the center of the brownie cup through the Oreo. It should come out with a few crumbs (not batter) sticking to it.
Once your brownies have finished cooking, remove the pan from the oven and allow them to cool and set for about 5 minutes before allowing to cool completely on a wire rack.
Notes
Store brownie cups in an airtight container at room temperature for up to 3 days.