Ōi Sadakiyo served as a bushō during the Sengoku period.  Sadakiyo served as the lord of Uchiyama Castle in the Saku District of Shinano Province.  He was a member of the Iwamurada-Ōi clan, an illegitimate branch of the Ōi clan who, in turn, were an illegitimate branch of the Ogasawara clan of the Shinano-Genji.

In 1541, Takeda Harunobu (Shingen) ousted his father, Takeda Nobutora, and took over the Takeda clan.  He then proceeded with the Invasion of Shinano.  In 1542, the Takeda subjugated the Suwa District.  On 9/17 of Tenbun 12 (1544), the Takeda toppled Nagakubo Castle in the Chiisagata District.  Sadakiyo’s adoptive father (and older brother), Ōi Sadataka, was apprehended by the Takeda and transported to Kōfu.  Sadakiyo succeeded to the family name of the Iwamurada-Ōi clan.

On 5/3 of Tenbun 15 (1547), when the Takeda army commenced an invasion of the Saku District, Sadakiyo resisted from Uchiyama Castle.  On 5/21, Uchiyama Castle was toppled and Sadakiyo fled to Nozawa.  On 4/16 of the following year, he surrendered to the Takeda.  On 7/18, Sadakiyo and his son, Ōi Sadashige, went to Kōfu to serve the Takeda while Uehara Iga-no-kami (Oyamada Toramitsu), a retainer of the Takeda, was appointed as the chamberlain of Uchiyama Castle.

After submitting to the Takeda clan, he was said to be in communication with Uesugi Norimasa, the deputy shōgun of the Kantō based in Kōzuke Province and Murakami Yoshikiyo, the lord of Katsurao Castle in Shinano Province.  In 1549, Sadakiyo mediated an alliance between the two parties.  On 2/13, he promised to allocate a stipend of 100 kan mon in Tōgō to Kobayashi Heishirō of the group from Kōzuke Province.  These maneuvers, however, did not go well and, in the end, he completely submitted to the Takeda clan.

On 3/10 of Tenbun 20 (1552), Sadakiyo was ordered by the Takeda clan to be stationed at Uchiyama Castle and replaced Uehara Iga-no-kami as the chamberlain of the castle.  On 9/20, he was removed from his position and Toramitsu returned to the role as chamberlain.  Thereafter, it appears that the Takeda seized his territory, and he went to serve in Kōfu.

In 1575, Sadakiyo and his son, Sadashige, deployed for the Battle of Nagashino and fought against the allied forces of Oda Nobunaga and Tokugawa Ieyasu, but were killed in action.