Oka Ietoshi served as a bushō during the Sengoku and Azuchi-Momoyama periods.  He was a retainer of the Bizen-Ukita clan.

Ietoshi was born as the son of Oka Sōbei.

Ietoshi served as a chief retainer of Ukita Naoie for a long time.  He is known as one of the Three Elders of the Ukita (along with Osafune Sadachika and Togawa Hideyasu).  The relationship between Ietoshi and a servant of Naoie named Oka Gōsuke is uncertain.

In 1573, Naoie delegated to Ietoshi the construction of Okayama Castle along with development of the town below the castle.  In 1582, he joined Hashiba Hideyoshi and, together with Ukita Tadaie (the younger brother of a different mother of Naoie), led the Ukita army on a deployment to Bitchū-Takamatsu Castle.  In 1588, upon the death of Sadachika who managed political affairs for the Ukita clan, Ietoshi was assigned as his successor in this role.

Ietoshi was deeply trusted by Toyotomi Hideyoshi and assigned as a deputy to Hideie’s successor, Ukita Hideie.  In 1592, while deploying to the Korean Peninsula for the Bunroku Campaign, he fell ill and died.  Known for his military prowess, over the course of his lifetime, he is said to have participated in over forty battles.

In certain military chronicles from Bizen, he is referred to as Toshikatsu, but based on authenticated sources, his name was Ietoshi.

He was succeeded by his son, Oka Echizen-no-kami.