Cal. Bus. & Prof. Code § 2275
Any person who held a physician's and surgeon's certificate under the jurisdiction of the Osteopathic Medical Board of California and a degree of doctor of medicine issued by a medical school located in the state at any time prior to September 30, 1962, and approved by either the Osteopathic Medical Board of California or the Medical Board of California at the time such degree was issued, who applied in writing to the Medical Board of California for permission to utilize his or her degree of doctor of medicine, shall be authorized to use the term or suffix "M.D." and the use shall not constitute unprofessional conduct, so long as the person advised both boards, in writing, that he or she has elected to use the term or suffix "M.D." and further has elected not to use the term or suffix "D.O." In the event of such election, the use of the term or suffix "D.O." constitutes unprofessional conduct within the meaning of this chapter.
Ca. Bus. and Prof'l. Code § 2275