My best friends father is an oral surgeon who does lots of cosmetics dealing with the jaw line and elongating chins, etc...
He said that plastic surgeons (MD) can put in a chin implant on a patient to move a chin (jaw line) up a few mm, but that an oral surgeon can actually take the jaw apart, add in pins and screws, and actually move a persons jaw forward to make their profile even (which implants cannot do in some cases). He also said many plastics refer patients to him to correct gum lines and such by rearranging the line of the jaw.
Besides that he deals with TMJ, cleft pallet, and said some perio and prostodontists in the area ask him to do a joint cases on complex implant cases where where they need to take bone from the sinus cavity since the patient does not have enough gum remaining in the gum area.
I am just a pre-dent so I personally do not know much about it, but I thought I would just tell you what my friend 's Dad said about it all. BTW- He does not do boob-jobs, etc... just sticks to oral, but he does makes big $$$