As foreign minister, Ōhira negotiated the principal agreement with the Republic of Korea (ROK, South Korea) treating compensation stemming from the Japanese occupation of Korea. He achieved this through direct talks with Kim Jong Pil, head of the Korean Central Intelligence Agency, in 1962. This agreement paved the way for the normalization of diplomatic relations with South Korea. Ōhira was also of assistance to Prime Minister Tanaka Kakuei in normalizing relations with the People's Republic of China (PRC) in 1972. In 1971 Ōhira took over leadership of the old Ikeda faction of the Liberal Democratic Party (LDP).
As prime minister from December 1978, Ōhira supported the study of traditional conservative Japanese values and in 1979 hosted the Tokyo Summit. Although considered a moderate in foreign policy, he sharply criticized the Soviet Union for its invasion of Afghanistan that same year. While in office, Ōhira died of a heart attack on 12 June 1980 in Tokyo.
Satō Seizaburō, Kōyama Ken'ichi, and Kumon Shunpei. Postwar Politician: The Life of Former Prime Minister Masayoshi Ōhira. Translated by William R. Carter. Tokyo: Kōdansha, 1990.