Preheat oven to 350°F. Line an 8-inch square pan with parchment paper, leaving an overhang on all four sides of pan. Lightly grease with cooking spray and set aside.
In a large bowl, add butter, oil, chopped unsweetened chocolate, and unsweetened cocoa powder. Fill a medium pot with about two inches of water. Bring to a boil and remove from heat. Place bowl over pot of water (making sure bowl does not touch water) and allow ingredients to gently melt. Stir as needed to ensure even melting. Add sugar and mix until thoroughly incorporated. Remove bowl from pot. Slightly cool mixture before stirring in eggs one at a time.
In another bowl, whisk together flour, salt, and baking powder. Add to melted chocolate mixture and mix until there are no longer any dry streaks of flour.
Reserve a handful of chopped Oreos and set aside. Add remaining chopped Oreo cookies to brownie batter. Fold to combine. Be careful not to over mix.
Pour batter into prepared baking pan. Smooth into an even layer. Top with reserved Oreo cookies. Bake for 28-30 minutes until the surface of the brownies are set and a toothpick inserted in center of brownies comes out clean.
Allow brownies to cool in pan for 15-20 minutes. Lift brownies out of pan using parchment overhang. Transfer brownies to wire rack to cool. Once cooled, slice brownies into desired portions. Store brownies are room temperature in an airtight container.