They will be very dry and unpleasant, but, you can substitute unsweetened applesauce for oil or butter in brownies. A tasty, healthier solution to your problem.
A half cup of melted butter is equivalent to a half cup of vegetable oil. Added: One stick of butter usually is the same as a half a cup but usually would need to add bit more since butter is...
Everything I have read suggests a one to one ratio of replacement, 1/2 cup of mayonnaise for 1/2 cup oil, for example. If you are out of oil and need an easy substitute, it sounds like a good one....