“What do you suggest for a child with oppositional defiant disorder who always has an intense need to have the last word?”

Unfortunately, dealing with a child who has this need to win often generates in parents the same intense need to come out on top.
Your strategy here would simply be to give your child the control he or she wants. Make the conscious decision to “surrender to win.” Go ahead and allow your child to have the last word.
Once his or her goal has been accomplished, the behavior usually stops. “Parting-shot” comments can be ignored and consequences given later (similar to the strategy outlined here).
Once his or her goal has been accomplished, the behavior usually stops. “Parting-shot” comments can be ignored and consequences given later (similar to the strategy outlined here).