Although the osteopathic profession emerged strong in California, D.O.s faced a setback as the California Medical Practice Act passed in 1907. In 1919, the composite board for this act refused to examine D.O.s and established a separate board of examiners. The push by both the California Medical (CMA) and California Osteopathic (COA) associations was towards a merger. They wanted to convert the College of Osteopathic Physicians and Surgeons (COPS) into an allopathic medicine college. In May of 1961, the associations signed a contract, and COPS became the California College of Medicine.

The new school gave M.D. degrees to its faculty and to the D.O.s who agreed to the conversion. D.O.s were able to obtain an M.D. degree by attending 12 Saturday classes and paying $65. A year later, Proposition 22 passed—abolishing the osteopathic licensing board. About 85% of practicing D.O.s traded in their D.O.s degrees for M.D. degrees. However, these M.D. degrees were not recognized outside the state.

What appeared to be a defeat for the profession, turned into a catalyst for obtaining full licensure in all 50 states. Because the COPS conversion took place with few curriculum changes and no additional training for D.O.s, osteopathic medicine became recognized as equal to allopathic. By 1973, all 50 states and the District of Columbia granted D.O.s full practice rights.

Simultaneously, the AOA and osteopathic physicians in California worked to overturn Proposition 22, and in 1974, the California Supreme Court declared the policy as unconstitutional. A new osetopathic board was established, and in 1977, the College of Osteopathic Medicine of the Pacific was opened.