Mpp,
Correction, you do not need a MD degree to practice Oral & Maxillafacial Surgery (OMS)! An OMS specialization is additional 4 years of training after 4 years of dental school (a DDS/DMD degree).
However you are correct about OMS/MD degree available to DDS/DMD degree holders, but this is a 6-year training program in addition to the 4 years in dental school!