Osafune Tsunanao served as a bushō and retainer of the Ukita clan during the Azuchi-Momoyama period. Tsunanao was the eldest son of Osafune Sadachika, a retainer of Ukita Naoie. He was the older brother of Osafune Sadayuki. Tsunanao received the title of Provincial Governor of Kii.
In 1591, Tsunanao’s father, Sadachika, was killed by Ishihara Shintarō, the husband of Sadachika’s younger sister. In this incident, Sadachika’s home base at Kokura Castle in Bizen Province was also burned down. Tsunanao was away at the time so avoided the calamity. It can be surmised these acts were plotted by the Ishihara family.
In 1594, Tsunanao served as a magistrate for the construction of Fushimi Castle in a strategic location between Ōsaka and Kyōto. Built on the plains, this was intended to serve as a retirement home for Toyotomi Hideyoshi. The construction was completed in 1596, but, soon thereafter, the main structure and associated temple buildings were destroyed in the Keichō-Fushimi Earthquake. In 1597, another castle was built approximately one kilometer to the northeast of Fushimi on Mount Kohata. Hideyoshi then died in this second location in 1598. After the initial construction of Fushimi, Hideyoshi recognized Tsunanao’s abilities and appointed him to administer the Ukita domain in lieu of Oka Ietoshi. In an effort to address the family’s financial problems, Tsunanao engaged in land surveys and other activities together with Nakamura Jirōbe-e (a close associate of Hideie who excelled in domestic affairs and contributed to the renovation of Okayama Castle) and Ukita Tarōsaemon Hidekuni (a commander of ashigaru, or lightly armed foot soldiers, under Ukita Naoie). This led to conflicts with Togawa Michiyasu and Oka Echizen-no-kami (Sadatsuna). Tsunanao’s conversion to Christianity further raised tensions with Michiyasu and Echizen-no-kami who were followers of the Nichiren sect of Buddhism.
Tsunanao died of illness either in 1597 or 1599, prior to which he is said to have reconciled with Michiyasu. Under an alternate theory, he was poisoned by Michiyasu and Echizen-no-kami. Tsunanao’s despotism was one of the factors leading to internal conflicts known as the Ukita Disturbance that beset the clan from early in 1599.