Ōno Harunaga (大野 治長, 1569 – 4 June 1615) was a general under Toyotomi Hideyori, and fought in the Siege of Osaka in 1615.

Afterwards, the pro-war faction within the Toyotomi family gained ascendancy and enlisted the services of ronin from various parts of the country to take part in the Siege of Osaka winter campaign however; Harunaga passively sued for peace, earning him the animosity of Sanada Yukimura's pro-war faction.

In 1615 during the summer campaign of the Siege of Osaka, Harunaga sent Senhime (daughter of Tokugawa Hidetada and Hideyori's wife) as a messenger for Ono, to plead for clemency for Hideyori and Yodo-dono, on the condition that he commit ritual suicide, but subsequently he and Hideyori committed suicide in a mountain hamlet in the vicinity of Osaka Castle.

Records show that rumours abounded throughout the Edo period regarding Harunaga having an adulterous affair with Yodo-dono, and some accounts maintain that Hideyori was not a biological child of Hideyoshi, but of Harunaga and Yodo-dono.

As described above, because he was a foster brother to Yodo-dono, it is likely that their relationship was extremely close; however, with the decline of the fortune of the Toyotomi family and the accompanying 'futility of ruin,' it can also be thought that the rumors of the sin of adultery were created with a view to casting him as an evil person (from a Confucian sense of resignation).