Ōdachi Harumitsu served as a bushō during the Sengoku period.  Harumitsu was a direct retainer of the shōgun under the Muromachi bakufu.

Harumitsu was born as the son of Ōdachi Hisauji, a bushō from the late Muromachi period.

Based on a customary practice in the family, Harumitsu received one of the characters in his name from Ashikaga Yoshiharu, the twelfth shōgun of the Muromachi bakufu, and adopted the name of Harumitsu.  Later, his younger brother, Fujiyasu, and his lineal heir, Terumitsu, each received a character in their name from Ashikaga Yoshiteru, the thirteenth shōgun.

The Ōdachi were a branch of the Nitta clan who, in turn, were of the same family as the Ashikaga clan.  For generations, the Ōdachi served in important roles for the Muromachi bakufu.  His father, Hisauji, was known as an expert in the affairs of the Imperial Court, the nobility, and military families, including laws, customs, and events.  Harumitsu’s sister was a consort of Ashikaga Yoshiharu, the twelfth shōgun, so he had positive experiences with the Ashikaga shōgun family.  In 1559, Harumitsu was conferred the title of Junior Fourth Rank (Lower) and held consecutive roles, among others, of Assistant Captain of Outer Palace Guards of the Left Division, Deputy Governor of Kazusa, and Governor of Mutsu.

Yoshiteru appointed Harumitsu to serve as a leader in negotiations with Nagao Kagetora (Uesugi Kenshin) of Echigo Province upon which Yoshiteru placed great importance.  In 1559, when Kagetora traveled to Kyōto, Harumitsu was assigned to host him.  Although unsuccessful, upon orders of Yoshiteru, Harumitsu served as an intermediary in peace negotiations between Kagetora and Hōjō Ujiyasu.  In the sixth month of 1563, during a visit to Kyōto by Mogami Yoshimori and Mogami Yoshiaki (father and son), Harumitsu served as a go-between for communications with the Muromachi bakufu and Imperial Court.

On 4/28 of Eiroku 8 (1565), Harumitsu died.  Soon thereafter, in an event known as the Eiroku Incident, Yoshiteru was killed by the combined forces of the Miyoshi and Matsunaga clans.  It has been confirmed that Harumitsu’s lineal heir, Ōdachi Terumitsu, served Ashikaga Yoshihide, but his whereabouts thereafter are unknown.  As a result, after Ashikaga Yoshiaki became the fifteenth shōgun with the backing of Oda Nobunaga, it appears that Terumitsu and members of the Ōdachi family at large were ousted from their positions and fell into ruin.