Ōuchi Sadatsuna served as a bushō from the Sengoku to early Edo periods.  He was the lord of Obama Castle in the Adachi District of Mutsu Province.  In the Edo period. he became a retainer of the Sendai domain.

In 1546, Sadatsuna was born as the lineal heir of Ōuchi Yoshitsuna.  In the era of Yoshitsuna, the Ōuchi clan responded to solicitations from the Tamura clan and ousted their lord, Ishibashi Hisayoshi.  The Ōuchi then became the landlords of Shiomatsu under the command of the Tamura clan.

He received one of the characters from the name of the Tamura clan and initially adopted the name of Akinori.  After Akinori inherited the headship of the Ōuchi clan, around the third month of 1579, the Tamura clan attacked the Iwaki clan.  This led to a dispute between the retainers of the Tamura and the Ōuchi.  Owing to his dissatisfaction with the resolution of this dispute, Akinori gradually plotted to become independent of the Tamura clan.  Upon learning of this plan, Nihonmatsu Yoshitsugu, the lord of Nihonmatsu Castle, mediated between Akinori and Tamura Kiyoaki but these efforts failed.  In the sixth month, Akinori proclaimed that he cut ties with the Tamura clan and changed his name from Akinori to Sadatsuna.  Having lost face owing to the failed mediation, Yoshitsugu sympathized with him.

Having become independent, Sadatsuna turned for support to Ashina Moriuji.  The Ashina clan obliged and the Date clan (who were allies of the Ashina) recognized the independence of Sadatsuna despite maintaining a neutral posture vis-à-vis the Tamura clan who were relatives of the Date.

In 1582, during the capture of Kosai Castle by Date Terumune, Sadatsuna visited Terumune’s encampment and came under the command of the Date.  Thereafter, he served in numerous battles against the Sōma clan.  Around this time, he arranged for his daughter to wed Yoshitsugu’s son, Kuniōmaru (later known as Nihonmatsu Yoshitsuna), to bolster his political standing.  In 1583, he proceeded to attack Ishikawa Mitsumasa, the lord of Dōmeki Castle, in the territory of the Tamura.  (Mitsumasa was the son of Ishikawa Arinobu, a former retainer of the Ishibashi clan who joined with Ōuchi Yoshitsuna to oust Ishibashi Hisayoshi.)  With the support of Ashina Moritaka (who opposed the Tamura), Sadatsuna defeated Tamura Kiyoaki.

In 1584, Terumune’s lineal heir, Date Masamune, inherited the headship of the Date clan.  (In 1579, Masamune received as his formal wife the daughter of Tamura Kiyoaki, Megohime.)  Sadatsuna then declared his continued service to the Date clan under Masamune.  Meanwhile, Ashina Moritaka and Nihonmatsu Yoshitsugu endeavored to gain the support of Masamune to forge a peace between the Ōuchi and Tamura clans.  In the eighth month, however, Masamune adopted a new policy to align with the Tamura clan.  To avoid a clash with Ashina Moritaka, Masamune refrained from attacking Sadatsuna, but in the tenth month, Moritaka was assassinated.  In the course of the ensuing succession struggle, those in the clan favoring subordination to the Date lost power and, as the Satake clan gained further influence, the alliance between the Date and Ashina clans unraveled.  Sadatsuna then made an impromptu visit to Masamune at Yonezawa Castle to propose that he serve the Date clan while his wife and children reside in Yonezawa.  Masamune accepted his proposal but after his return to Shiomatsu, his father, Yoshitsuna, spurned the agreement.  This enraged Masamune who then joined forces with the Tamura clan and decided to attack Sadatsuna.  Although retired, Terumune covertly requested Sadatsuna to apologize to Masamune but Sadatsuna did not respond.

In the fifth month of 1585, Masamune attacked the Ashina clan for supporting Sadatsuna.  In the eighth month, he attacked Sadatsuna and made a clean sweep of Odemori Castle, killing everyone.  Sadatsuna abandoned his base at Obama Castle and fled for safety to Nihonmatsu.  He then turned to the Ashina clan in Aizu for protection.  In 1588, at the Battle of Kōriyama, Sadatsuna served as a unit commander for the Ashina clan in the capture of Nawashiroda Castle but was then lured by Date Shigezane and, together with his younger brother, Katahira Chikatsuna, reverted to the service of the Date owing to frustration at the cold treatment that he received from the Ashina.  After learning that Chikatsuna’s base at Katahira Castle was in a vital location for an attack by Masamune on Aizu and a group of rōnin in Shiomatsu (former retainers of the Ōuchi and Nihonmatsu clans) continued their resistance against the Date, he benefited from a favorable position from which to commence negotiations with Masamune.  After the fall of Sadatsuna, rumors circulated of a rebellion by Ishikawa Mitsumasa after he entered Shiomatsu.  (In fact, after Sadatsuna reverted to the Date, the Ishikawa were lured by the Sōma and did rebel.)  In the third month of 1588, Sadatsuna was permitted to return to the Date but instead of his former territory, he was granted landholdings in the Date and Nagai districts near the base of the Date clan.  Owing to military pressure from the Ashina clan, Chikatsuna delayed returning to the Date but in the third month of 1589, he returned after receiving recognition of his rights to his landholdings.  Based on these developments, the mother of Sadatsuna and Chikatsuna held as a hostage was killed by Ashina Yoshihiro.

Thereafter, Sadatsuna participated in the Battle of Suriagehara, the suppression of the Kasai-Ōsaki Uprising, and the Bunroku-Keichō Campaign.  In 1591, after Masamune was transferred to Iwadeyama Castle, Sadatsuna was granted landholdings of approximately 10,000 koku in the Isawa District and became the lord of Maesawa Castle.  In 1600, during the Battle of Sekigahara, he watched over the Date residence in Kyōto.  Owing to these contributions, in the era of his son, Ōuchi Shigetsuna, the Ōuchi were granted the status of family members of the Date.

Character

Sadatsuna was highly regarded for his military prowess, excelled in stratagems, and skilled in the use of the jūmonji (a spear with curved cross-blades) and associated fighting techniques.