I'm not exactly sure what you're referring to b/c every dentist and specialist makes a diagnosis when s/he has a patient in the chair. Maybe if you could be a little more specific, we could help you with more info.
I think Oral Diagnosis could also be called Oral Pathology which is an ADA recognized specialty. There was a thread on this a week or two ago.
there are residencies available in Oral Medicine/Diagnosis. This training gives the dentists more understanding about systemic diseases that affect the oral caivty (i.e. Pemphigus Vulgaris, Bullous Pemphigoid, Lupus, etc.) The oral medicine specialist treats the oral conditions of these individuals. It is not an ADA recognized specialty. It seems interesting, however, most of the prgrams are 3 years, and the extra training will probably land you in an academic environment. There seems to be (SEEMS TO BE) not a big need for these kind of dentists in private practice. There is an American Academy of Oral Medicine website that publishes a list of residency programs and info. Can't remember the address. Sorry!
You'll see a big variety of oral pathologists/oral diagnosis Docs where you're going to dental school(UCONN if I'm not mistaken). You have everything from those that specialize in treating/managing the oral manifestations/side effects of chemotherapy treatments (Dr's Peterson and Nuki) to anti caries/ anti Sorgrens Syndrome research specialist (Dr Tanzer) to more "traditional" oral diagnosis Docs dealing with soft/hard tissue manifestations of oral diseases and oral pathology analysis (Dr's D'Ambrosio and Eisenberg). All and all, that department can be a bit ecclectic at times, but as a whole they're a great bunch of people and a tremendous resource (I still call them up once or twice a year with patient referrals.