Emperor Ōjin (応神天皇) was the 15th imperial ruler of Japan to appear on the traditional list of emperors. No firm date can be assigned to this emperor.

He is the earliest historical emperor. He has been deified as Hachiman Daimyojin, regarded as the guardian of warriors. The Minamoto Clan considered him their guardian Kami.

According to the Kojiki and the Nihonshoki, Ojin was the son of the Emperor Chuai and the Empress Jingo. As Chuai died before Ojin's birth, his mother Jingo became regent. Ojin was born in 200 on the return of his mother from a mythological invasion of Korea and named Prince Homutawake. He became the crown prince at four years old. He was crowned in 270 at 70 years old and reigned for 40 years until his death in 310, although all the dates around his reign have no historical basis. He lived in two palaces both of which are in Osaka today.

He was recorded as the father of Nintoku, his successor.