Oniniwa Yoshinao served as a bushō during the Sengoku and Azuchi-Momoyama periods.  He was a retainer of the Date clan, serving four generations of heads of the clan.

In 1513, Yoshinao was born as the son of Oniniwa Motozane, the lord of the Koya-date (Aka-date) residence in the Date District of Mutsu Province.

In 1539, upon the retirement of his father, Motozane, Yoshinao inherited the headship of the clan.  In 1542, upon the outbreak of the Tenbun Conflict, Yoshinao and Motozane fought on the side of Date Harumune.  Owing to his contributions, in 1549, he became the lord of Kawai Castle in the Okitama District of Dewa Province.  His landholdings were increased by 2,000 koku.  According to one account dated in 1553, these landholdings were held in the name of Motozane.

In 1549, his consort (the daughter of Makino Osakabe), bore a long-awaited son, Oniniwa Tsunamoto.  To ensure recognition of Tsunamoto as his lineal heir, he divorced his formal wife, Naoko, on grounds that she did not bear a son and expelled her and their daughter, Kita, from the Oniniwa family.  He then raised the status of his consort to that of his formal wife.

In 1564, after Date Terumune became the head of the Date clan, Terumune appointed Yoshinao to a consultative role as he sought his close associates to acquire more personnel.  Consequently, Yoshinao, together with Endō Motonobu, formed the nucleus of Terumune’s administration.  Yoshinao performed his role faithfully and, at the beginning of the Tenshō era (1573 to 1593), the Oniniwa were granted the status of members of the Date family.  In 1575, Yoshinao transferred headship of the clan to his son, Tsunamoto, and retired, adopting the monk’s name of Sagetsusai.  Even after retiring, he continued to serve as a close associate of Terumune and engaged in political affairs.  In 1577, when Terumune’s younger brother, Masashige (later known as Kokubun Morishige), became the heir of the Kokubun clan, Yoshinao negotiated the arrangements.

In the tenth month of 1585, after having transferred headship of the clan to his lineal heir, Date Masamune, Terumune was murdered.  On 11/17 of Tenshō 13 (1586), Satake Yoshishige of Hitachi Province gave up on Masamune and rallied lords from the southern portion of Mutsu who had abandoned the Date clan and amassed in the Adachi District to attack the Date army laying siege to Nihonmatsu Castle.  This is known as the Battle of Hitotoribashi.  At this time, Masamune delegated command to Sagetsusai and gave him a gold-colored baton.  Outnumbered, however, the Date army fell into disarray, allowing the Satake army to penetrate their main base.  To enable Masamune to escape, Sagetsusai led the rear guard to attack the enemy forces.   Sagetsusai was elderly so he could not bear the weight of full armor.  Instead of a helmet, he wore a yellow cotton hat.  Nevertheless, the Oniniwa battalion held their ground on the front lines and took over 200 enemy heads.  During this time, Masamune narrowly escaped to Motomiya Castle.  Meanwhile, Sagetsusai was killed in action by a retainer of Iwaki Tsunetake named Kubota Jūrō.  He was seventy-three years old.

To recognize the contributions of Sagetsusai, Masamune granted his landholdings to his widow and issued a license to officially recognize her rights to this property for the rest of her life.