Preheat oven to 350°C/180°C, and lightly grease a 9x9 inch baking pan (22x22cm).
In a large bowl, combine the oil with sugar. Add the eggs and vanilla extract, and whisk again until well combined.
In a separate bowl, combine the dry ingredients (flour, cocoa powder, baking powder, and salt).
Add the dry ingredients to the wet ingredients, and mix until just combined. Do not overmix the batter.
Gently fold in ¾ of the crushed Oreo cookies, and pour the batter into the prepared pan.
Sprinkle the rest of the Oreos over the top of the brownies.
Bake for 20 minutes or until toothpick comes out clean from the center of the brownies.
Cool completely before slicing. Slice into squares, and serve.