Preheat the oven to 350 degrees F and prepare an 8x8 baking pan with nonstick cooking spray and parchment paper.
In a microwave-safe bowl, combine unsalted butter and brown sugar. Heat in the microwave for 90 seconds until the butter is fully melted. Whisk the butter and brown sugar together until combined and smooth. Set aside.
In a large mixing bowl, add eggs and granulated sugar. Use an electric hand mixer to whip the eggs and sugar on medium-high speed for a few minutes until light and pale in color.
Slowly add the melted butter mixture into the egg mixture while mixing on medium speed with the hand mixer to temper the eggs. Once all the butter mixture is added, add in vanilla extract and vegetable oil and mix until combined.
Add in salt, baking powder, and dark cocoa powder and mix on low speed until well combined. Scrape the sides and bottom of the bowl.
Add in all-purpose flour and mix on low speed until the flour is fully incorporated into the chocolate mixture.
Fold in 2 cups of roughly chopped Oreo cookies until well dispersed throughout the batter.
Pour the brownie batter into the prepared baking pan. Use an offset spatula to spread the batter evenly in the pan. Press additional Oreo cookies into the top of the brownie batter, if desired.
Bake the brownies for approximately 30-35 minutes until the center of the brownies is set. To check for doneness, insert a toothpick into the center of the brownies. The toothpick should come out with moist crumbs, not wet brownie batter. If there is wet brownie batter, bake for a few more minutes.
Remove the pan from the oven and cool the brownies in the pan to room temperature. Remove the brownies from the pan and cut into squares.