Actually, the reason that DO to MD carribean conversion program works is because if you graduate from ANY WHO listed school, no matter what their degree is, you can be an MD
In the United States, regardless of where you went to school and what degree you received, so long as the school is listed on WHO and so long as you go through the ECFMG process to obtain licensure by taking all steps of the USMLE and securing residency... you can call yourself an MD .
Example: in the UK, England, Australia, India, systems, the actual degree conferred is MBBS, which is a bachelors of medicine and a bachelors of surgery. its not even truly a doctoral degree.
Other countries have the MBChM, BSBM, etc. The degrees conferred ARE NOT MD, yet an FMG from UK or India with an MBBS degree can and will write the letters M.D. after their name in the US. This is because the designation MD is doctor of medicine actually based on the licensed write to practice and not the degree.
On the osteopathic side, the osteopathic physicians and the AOA have for whatever reason lobbyed hard to prevent being recognized or accredited by other organizations including WHO and LCME so that they can maintain the exclusive ability to recognized and regulate their own. Technically speaking however, if US DO schools were listed on the WHO, then their grads could essentially go through the process of ECFMG licensure, take USMLE steps I, II, III and in essence call themselves MD's. The AOA does not want this because its their way to maintain identity and i individuality as osteopathic profession.