It depends on what kind of doctor, and at times his/her specialty. For example, A general physician can be an MD, or DO. A doctor of psychology could be a PsyD or PhD, a dentist can be DDS, etc.
OD are the initials after an optometrists name. It stands for Doctor of Optometry. Yes, it is true that DO is for Doctor of osteopathy, but OD is for Doctor of Optometry contrary to the imbecile who...
They tell you what the doctorate is in. M.D. is medical doctor, D.C. is doctor of chiropractic, Ph.D. is philosophy doctor - which could be almost anything - business, history, etc., Sci.D. is...