Oai -- likely given the formal name Masako at birth -- grew up in eastern Mikawa Province during the most violent era in Japanese history. Her father, Tadaharu, served as a retainer under the powerful Imagawa clan. In 1554, when Oai was just two years old, Tadaharu was killed at the Battle of Enshu-Omori, fought between the Imagawa and the Hojo clan. Her mother remarried two years later to Hattori Masanao, producing four more children, only two of whom survived to adulthood. As a young woman, Oai married into the Saigo clan through Saigo Yoshikatsu, and bore two children: a son, Saigo Katsutada, around 1570, and a daughter. But the wars followed her. In 1571, Yoshikatsu was killed at the Battle of Takehiro, fighting invading Takeda forces. Widowed with two small children, Oai was formally adopted by her uncle, Saigo Kiyokazu, the head of the Saigo clan. The adoption gave her the Saigo surname she would carry for the rest of her life.

Tokugawa Ieyasu first noticed Oai while she was still married, and nothing came of it. After her widowhood, a genuine friendship developed between them during the 1570s. Unlike the political marriage that bound Ieyasu to his first wife, Lady Tsukiyama -- a union marked by jealousy, tempestuous moods, and arranged by the Imagawa clan -- his relationship with Oai appears to have been chosen freely. Starting around the time of the Battle of Mikatagahara in 1573, Ieyasu began confiding in her and seeking her counsel. She is credited with advising him as the Battle of Nagashino approached in 1575, a pivotal engagement that broke the power of the Takeda cavalry and reshaped the trajectory of Japanese warfare. Ieyasu continued seeking her advice on battles and alliances through the Komaki-Nagakute Campaign of 1584. In a world where concubines were typically valued for beauty and fertility, Oai earned her place through strategic intelligence.

Lady Saigo bore Ieyasu two sons. The first, born in 1579, was Tokugawa Hidetada, who would become the second shogun of the Tokugawa dynasty. The second, born on October 18, 1580, was Matsudaira Tadayoshi. That same year, Lady Tsukiyama was executed on suspicion of conspiring with the Takeda clan against Ieyasu's ally Oda Nobunaga. Ieyasu's first son by Lady Tsukiyama, Tokugawa Nobuyasu, was ordered to commit seppuku. With their deaths, Hidetada became heir apparent, and Lady Saigo's position at court was unassailable. She used that security for philanthropy. A devout Buddhist, she donated to temples across Suruga Province. Being severely near-sighted herself, she founded a charitable organization to support visually impaired women who had no other means of livelihood -- an act of institutional compassion remarkable for sixteenth-century Japan.

Shortly after taking up residence at Sunpu Castle, Lady Saigo's health began to fail. She died on July 1, 1589, at the age of thirty-seven. The official account attributed her death to physical and emotional hardship, but murder was suspected. No culprit was ever identified. She was interred at the temple Ryusen-ji. Ieyasu did not forget her. After completing his unification of Japan and being named shogun in 1603, he had Ryusen-ji relocated to Koyamachi near Sunpu Castle. He attended Buddhist funeral rites on the anniversary of her death and presented the temple priests with the katana he had inherited from his own father, along with a portrait of himself. He renamed the temple Hodai-in, incorporating characters from her posthumous Buddhist name. The sword and portrait remain viewable there in modern Shizuoka city. Lady Saigo's great-granddaughter, Princess Okiko, ascended the Chrysanthemum Throne as Empress Meisho in 1629 -- the seventh of only eight women to reign as empress in the entire history of Japan. She ruled for fifteen years as the 109th monarch. The girl from Mikawa called Oai had shaped a dynasty that reached from the shogun's court to the imperial throne.