Preheat oven to 350 F and lightly grease a 9 x 9 baking sheet.
Lightly combine flour, cocoa powder, salt, and baking powder.
In a large mixing bowl, whisk together oil and sugar until well blended, then add eggs and vanilla, whisking until just incorporated.
Add the dry ingredients to the wet, then stir until just combined.
Pour half of the mixture into the baking dish, then lay on the Oreos. Be sure to leave some for the topping.
Pour in the rest of the batter. Bake until the sides are beginning to pull away from the pan, about 20 minutes. It may not quite pass the toothpick test, but it'll be close. While still warm, top with broken Oreos, lightly pressing so they mush into the warm brownies.