Lady Saigō (西郷局 or 西郷の局 Saigō no Tsubone; 1552 – 1 July 1589), also known as Oai, was one of the concubines of Tokugawa Ieyasu, the samurai lord who unified Japan at the end of the sixteenth century and then ruled as shōgun. She was also the mother of the second Tokugawa shōgun, Tokugawa Hidetada. Her contributions were considered so significant that she was posthumously inducted to the Senior First Rank of the Imperial Court, the highest honor that could be conferred by the Emperor of Japan.

During their relationship, Lady Saigō influenced Ieyasu's philosophies, choice of allies, and policies as he rose to power during the late Sengoku period, and she thus had an indirect effect on the organization and composition of the Tokugawa shogunate. Although less is known of her than some other figures of the era, she is generally regarded as the "power behind the throne", and her life has been compared to a "Cinderella story" of feudal Japan.

Once she was in a respected and secure position as the official concubine and mother to Ieyasu's heir, Lady Saigō used her influence and wealth for charitable purposes. A devout Buddhist, she donated money to temples in Suruga Province, where she resided as the consort of Ieyasu, first in Hamamatsu Castle and later in Sunpu Castle. As she was quite near-sighted, she also established a charitable organization that assisted visually impaired women with no other means of support. Lady Saigō died at a fairly young age, under somewhat mysterious circumstances. Although murder was suspected, no culprit was identified.

Lady Saigō bore four children: she had a son and a daughter (Saigō Katsutada and Tokuhime) while married, and she later bore two sons as the consort of Tokugawa Ieyasu: Tokugawa Hidetada and Matsudaira Tadayoshi. Among the descendants of Lady Saigō was the Empress Meishō (1624–1696), one of very few women to accede to the Chrysanthemum Throne as empress regnant.

The term "Saigō-no-Tsubone", used in most historical texts, is an official title rather than a name. As an adult she was adopted into the Saigō clan, so she was permitted to use the surname. Later, when she was named first consort of Tokugawa Ieyasu, the title "tsubone" (pronounced [tsɯbone]) was appended to the surname. The title was one of several titular suffixes conferred on high-ranking women (others include -kata and -dono). The bestowal of a title depended on social class and the relationship with her samurai lord, such as whether she was a legitimate wife or a concubine, and whether or not she had had children by him. The word tsubone indicates the living quarters reserved for ladies of a court, and it became the title for those who had been granted private quarters, such as high-ranking concubines with children. This title, tsubone, was in use for concubines from the Heian period until the Meiji period (from the eighth century to the early twentieth century), and is commonly translated to the English title "Lady".

Though Lady Saigō's given name does not appear in surviving documents from the time, there is good evidence it was Masako (昌子), but this name is very rarely used. Her most commonly used name was Oai (お愛 or 於愛, meaning "love") and most sources agree this was a nickname she gained as a child. Intimate friends and family would call her Oai throughout her life, and it is the name most often used in modern popular cultural references. Following death, she was bestowed with a Buddhist posthumous name, and an abbreviation of that name, Hōdai-in (宝台院), is sometimes used out of pious respect.

The Saigō family was one branch of the distinguished Kikuchi clan of Kyushu that had migrated northward to Mikawa Province in the fifteenth century. In 1524, the forces of Matsudaira Kiyoyasu (1511–1536), the grandfather of Tokugawa Ieyasu, stormed and took the Saigō clan's headquarters at Yamanaka Castle during his conquest of the Mikawa region. Shortly after the battle, Saigō Nobusada, the third head of the Saigō, submitted to the Matsudaira clan. Following the untimely death of Kiyoyasu in 1536, and the ineffectual leadership and early death of Matsudaira Hirotada (1526–1549), the leaderless Matsudaira clan finally submitted to Imagawa Yoshimoto (1519–1560) of Suruga Province, east of Mikawa. When the Matsudaira fell to the Imagawa, the clans of their retainers, which included the Saigō, likewise submitted to the Imagawa. Following the Battle of Okehazama (1560), Saigō Masakatsu attempted to re-assert the independence of the clan while yielding some land concessions to the Imagawa. In response, Imagawa Ujizane arrested thirteen Saigō men, and had them vertically impaled near Yoshida Castle. The executions did not deter the Saigō, and in 1562 the Imagawa launched punitive invasions of east Mikawa and attacked the two main Saigō castles. Masakatsu was killed in the battle of Gohonmatsu Castle; his eldest son Motomasa was killed during the battle for Wachigaya Castle. Clan leadership passed to Masakatsu's son, Saigō Kiyokazu (1533–1594), who pledged his loyalty to the Matsudaira clan, under the leadership of Tokugawa Ieyasu, in their mutual struggle against the Imagawa. In 1569, the power of the Imagawa ended with the siege of Kakegawa Castle.

Neither the name of Lady Saigō's mother nor her dates of birth or death are recorded in any existing documents, although it is known that she was the elder sister of Saigō Kiyokazu. Lady Saigō's father was Tozuka Tadaharu [ja] of Tōtōmi Province, under direct control of the Imagawa clan. The marriage between Tadaharu and his wife was very likely arranged by the Imagawa clan.