Preheat your oven to 350°F (175°C). Line a 9x9-inch baking pan with parchment paper, making sure the paper hangs over the edges.

Pour the melted butter into a large mixing bowl. Add the vegetable oil and sugar, stirring until smooth and combined.

Add the eggs and egg yolk one at a time, whisking completely after each addition. Stir in the vanilla extract.

Sift in the flour, cocoa powder, and salt. Gently fold the dry ingredients into the wet mixture using a spatula or wooden spoon. Mix just until combined, avoid overmixing.

Stir in half of the chocolate chips and half of the chopped Oreos, making sure they are evenly distributed throughout the batter.

Pour the batter into the prepared pan and smooth out the top with a spatula. Sprinkle the remaining chocolate chips and chopped Oreos over the top. Press them gently into the batter but don’t submerge them completely.

Bake the brownies in the preheated oven for 35-40 minutes. The edges should be firm, and the center should still look slightly underbaked. A toothpick inserted in the center should come out with a few moist crumbs.

Remove from the oven and cool the brownies in the pan for at least 20 minutes. Use the parchment paper to lift the brownies onto a cutting board, then slice and serve. Enjoy!