[1] In the 1942 election Ōno sided with his mentor Hatoyama in refusing to join the Imperial Rule Assistance Association, running as a "non-recommended" candidate and thereby losing his seat in the Diet.

When Hatoyama and Liberal Party secretary-general Ichirō Kōno were purged by the US Occupation, Ōno stepped in as secretary general from 1946 to 1948, assisting Shigeru Yoshida during his first stint as prime minister.

[1] Having recovered his reputation, Ōno served as Speaker of the House of Representatives from 1952 to 1953, and then joined the Yoshida cabinet as Minister of State and Director of the Hokkaido Development Agency in 1953.

In particular, Kishi was angry that one of the main guarantors of the deal, Kōno, had openly opposed him during the Anpo protests of 1960, with tacit support from Ōno, while only his own brother Satō remained by his side.

Although this plan fell through after much discussion, with the latter three ultimately refusing to join Kōno in exiting the party,[7] by this time Kishi had come to view Ōno as one of the rebels that was undermining his government, and felt that the original terms of the secret agreement had been invalidated.

On July 14, 1960, Kishi was attacked by a knife-wielding assailant as he was leaving the prime minister's residence to host a garden party celebrating Ikeda's impending ascension to the premiership.