Ōishi Sadahisa was a retainer of the Ogigayatsu-Uesugi

Uesugi clan

The was a Japanese samurai clan, descended from the Fujiwara clan and particularly notable for their power in the Muromachi and Sengoku periods ....

 branch of the Uesugi, and the builder of Takiyama Castle

Takiyama Castle

Taki-no-jō Castle, , is a flatland-hilltop Japanese castle ruin located in Tokorozawa, Saitama Prefecture. It was on a key route between the northern Kanto Plain and Hachioji....

 (滝山城). After the Uesugi were defeated at Kawagoe in 1545, Ōishi accepted the authority of the Hōjō

Late Hojo clan

The ' was one of the most powerful warrior clans in Japan in the Sengoku period and held domains primarily in the Kantō region.The clan is traditionally reckoned to be started by Ise Shinkurō, who came from a branch of the prestigious Ise clan, a family in the direct employment of the Ashikaga...

. However, their leader was murdered, and was replaced by Hōjō Ujimasa

Hojo Ujimasa

was the fourth head of the late Hōjō clan, and daimyo of Odawara.Ujimasa commanded in many battles, consolidating his clan's position, and retired in 1590. His son Hōjō Ujinao became head of the clan and lord of Odawara, but later that year they failed to hold Odawara against the forces of Toyotomi...

's 2nd son Hōjō Ujiteru

Hojo Ujiteru

' was a Japanese samurai, who was the son of Hōjō Ujiyasu and lord of Hachiōji Castle in what is now Tokyo.Ujiteru commanded a major force at the battle of Mimasetoge, where he unsuccessfully attempted to prevent Takeda Shingen from withdrawing to his home province of Kai after sieging the Hōjō's...

. After the fall of the Hōjō, Ōishi Yoshinaka and Sadakatsu came to work for Okubo Nagayasu

Okubo Nagayasu

was a Japanese samurai of the Edo period who served the Tokugawa clan. He worked as a mining magistrate in various Tokugawa-controlled mines. Due to misconduct, he was deprived of office, and died soon afterward.-References:*http://fine-vn.com/cat_11/ent_60.html...

.