In a large bowl, cream together the brown sugar, white sugar and vegetable oil until light and fluffy. Add the eggs one at a time, beating well with each addition, then stir in the vanilla. Combine the flour, baking powder and salt; stir into the creamed mixture, then mix in the rolled oats. Cover and chill for at least 2 hours.
Preheat oven to 180 degrees C. Grease baking trays.
Roll dough into walnut sized balls, roll the balls in the icing sugar to coat and place 5cm apart onto the prepared baking trays.
Bake for 8 to 10 minutes in the preheated oven. Allow biscuits to cool on baking tray for 5 minutes before removing to a wire rack to cool completely.