Can an oral/maxillofacial surgeon also be a practicing anesthesiologist?

Not really: Although it was a dentist, horace wells who first performed general anesthesia and oral & maxillofacial surgeons have very advanced training in general anesthesia we are not anesthesiologists. However, during times of war the military have used oms's as surgeons and anesthesiologists because of our dual training.

Yes: There are a number of people who graduated from dental school, completed an oral and maxillofacial residency, medical school and a residency in anesthesia who are now anesthesiologists. Even though they may have a dental/oral and maxillofacial training, many of these individuals may limit their practice to anesthesia.