Ōba Sanzaemon served as a bushō during the Sengoku and Azuchi-Momoyama periods. He was a retainer of the Ashina clan. He is known as the individual who murdered his lord, Ashina Moritaka, the eighteenth head of the Ashina clan and sengoku daimyō of Mutsu Province.
Sanzaemon was the son of the chief priest at the Suwa Shrine in Sukagawa.
Initially, he served Nihonmatsu Yoshitsugu, a bushō, the ninth head of the Nihonmatsu clan, and lord of Nihonmatsu Castle in the Adachi District of Mutsu. Later, Moritaka received Sanzaemon from the Nihonmatsu clan and made him one of his retainers. Cherished by Moritaka, he was responsible for preparing swords for wearing by his lord. He was not, however, popular within the Ashina family.
On 10/6 of Tenshō 12 (1584), at Aizu-Kurokawa Castle, while his lord, Moritaka, was sitting in the external corridor on the outer side of the house offering feed to a falcon, Sanzaemon approached from behind and suddenly slayed him with a sword. After the killing, Sanzaemon attempted to flee, but was pursued by another servant, and was subsequently cut-down by a retainer of Moritaka named Tanehashi Daizō.
This event had significant consequences. The chronicles of the Date family include references to the murder of Moritaka. Satake Yoshishige (the eighteenth head of the Satake clan and sengoku daimyō of Hitachi Province) notified Yabe Shimotsuke-no-kami, warning him to remain vigilant on the provincial border. A letter from Iwaki Tsunetaka (the seventeenth head of the Iwaki clan and a daimyō in southern Mutsu) addressed to Ōuchi Sadatsuna (the lord of Obama Castle in the Adachi District of Mutsu) refers to the incident of the murder of Moritaka and includes a reminder to keep bow and arrow at the ready in the same manner as when Moritaka was alive.
He was a handsome young man so there is a story that the murder was connected to a homosexual relationship with Moritaka.
Sanzaemon excelled in the military arts, and, while serving Nihonmatsu Yoshitsugu, he was praised for his valor. Owing, however, to his skills, questions are raised as to whether he was so easily killed. Moreover, the details of his demise including whether he engaged in sword fight against Tanehashi Daizō remain uncertain.
When transferred from the Nihonmatsu clan to serve as a retainer of Moritaka, Sanzaemon was purportedly thirteen years old, but this is open to question.