Adjust oven rack to lower middle position and heat oven to 350 degrees. Line a 11x9 glass pan with parchment paper.
Melt chocolate and butter in medium heatproof bowl set over sauce pan of simmering water, stirring occasionally, until smooth. Whisk in cocoa until smooth. Set aside to cool slightly.
Chop the oreos into quarters and set aside.
Whisk eggs, sugar, vanilla, and salt together in medium bowl until combined, about 15 seconds.
Whisk warm chocolate mixture into egg mixture. Using rubber spatula, stir in flour until just combined. Leave a few streaks of flour not mixed in.
Stir in the oreos, leaving some to place on top of the batter.
Transfer the batter to a prepared pan. Spread the batter into the corners until smooth. Place the remaining oreos quarters on top of the batter, pressing down slightly.
Bake until slightly puffed and toothpick inserted in center of brownies comes out with a few moist crumbs attached, about 30 to 35 minutes.Let them cool in pan on a wire rack. Once cooled, lift the parchment paper out, and cut the brownies into squares.