Osafune Sadachika served as a bushō during the Sengoku and Azuchi-Momoyama periods.  Sadachika was a retainer of Ukita Naoie, the sengoku daimyō of Bizen Province.

The Osafune originated from the well-known Kai-Genji clan with ties to the Ogasawara clan.  Sadachika served as one of the Three Elders of the Ukita (along with Togawa Hideyasu and Oka Ietoshi), contributing to the governance of Ukita Naoie.

Sadachika served as a retainer from the early stages of Naoie’s rule, participating in battles against Saisho Mototsune and others.  At the Battle of Myōzenji against Mimura Motochika of Bitchū, Sadachika may have killed Shō Motosuke.  In conflicts against the Mimasaka-Miura, Sadachika made significant contributions in attacks on Uragami Munekage, a former lord of Naoie, at the Siege of Tenjinyama Castle.  At the Siege of Kōzuki Castle, upon orders of Naoie who had allied with the Mōri, Sadachika defeated Kuroda Yoshitaka. Yoshitaka was aligned with Hashiba Hideyoshi, the senior commander sent by Oda Nobunaga to support the defenders at Kōzuki.

After Naoie followed the advice of Hideyoshi to abandon the Mōri in favor of the Oda, Sadachika obeyed orders from Naoie to fight against the Mōri.  After Naoie ousted an influential kokujin, or provincial landowner, known as Iga Hisataka, Sadachika was assigned to Kokura Castle in Bizen.

Following the death of Naoie, Sadachika served under Ukita Hideie, serving as a proxy for Hideie in the Conquest of Shikoku.  After the retirement of Togawa Hideyasu, Sadachika managed provincial affairs for the Ukita.  On 1/6 of 1591, the husband of his younger sister, Ishihara Shintarō, took issue with Sadachika’s governance, killed him and burned down Kokura Castle.