Technically they are doctors, but of the dental variety. Optometrists are "Doctors of Optometry). They all are considered doctors of their field even though they are not "Medical Doctors". It is absolutely appropriate to refer to them as Dr. Smith (or whatever their last name is) as they have earned that title.
That being said, if they were to be asked their profession and they said they were a doctor that would probably be a little misleading as most people associate that "profession" as being a medical doctor.