Hosokawa Naomoto served as a bushō during the Sengoku period. He was the lord of Oguni Castle in the Mogami District of Dewa Province. Naomoto is surmised to have been a member of the Mutsu-Hosokawa clan and associated with the Hosokawa serving as the deputy shōgun for the Ashikaga clan of the Muromachi bakufu, but his origins are uncertain.
While governing Oguni in the Mogami District of Dewa, Naomoto cooperated with an alliance of powerful kokujin, or provincial landowners, led by the Tendō clan and known as the Eight Shields of the Mogami (also known as the Eight Shields of the Tendō). As a result, he soon came into conflict with Mogami Yoshiaki. In 1574, during the Tenshō Mogami Conflict (a struggle between Mogami Yoshimori and his eldest son, Mogami Yoshiaki), Naomoto aligned with Mogami Yoshimori (backed by Date Terumune), but was defeated and surrendered.
In 1581, Yoshiaki held a cavalry parade at Tendō-ga-hara, but, after having wed his daughter to Tendō Yorizumi who was opposed to Yoshiaki, Naomoto did not participate in the event. This was interpreted as a declaration of war against Yoshiaki, whereupon Yoshiaki ordered his retainer, Kurazō Akitada, to attack him. A Mogami army comprised of 3,500 soldiers led by Akitada traversed the Natagiri Pass to launch an assault against Naomoto in Oguni, triggering the Battle of Maginohara.
The Hosokawa, under the command of Naomoto and his younger brother, Hosokawa Naoshige, joined the fight with 350 troops. At a loss for how to handle a Mogami army ten times its size, the Hosokawa could not hold on to Oguni Castle and the members of the Hosokawa family were obliterated. Oguni was awarded to Akitada. Later, Akitada’s son, Akimoto, drew from the place-name to adopt the name of Oguni Akimoto.