While the historical existence of Emperor Ōjin is debated among historians, there is a general consensus that he was "probably real".
If Ōjin was an actual historical figure then historians have proposed that he ruled later than attested.
Dates of his actual reign have been proposed to be as early as 370 to 390 AD, to as late as the early 5th century AD.
At least one Japanese historian has cast doubt on this theory though, by revising a supporting statement given in 1972.
In this new narrative Louis Perez states: "only kings and emperors after the reign of Ojin... ...are seen as historical figures".
In either case there is also no evidence to suggest that the title tennō was used during the time to which Ōjin's reign has been assigned.
It is certainly possible that he was a chieftain or local clan leader, and that the polity he ruled would have only encompassed a small portion of modern-day Japan.
The name Ōjin-tennō was more than likely assigned to him posthumously by later generations.

While the actual site of Ōjin's grave is not known, this regent is traditionally venerated at a kofun-type Imperial tomb in Osaka.
The Imperial Household Agency designates this location as Ōjin's mausoleum, and is formally named Eega no Mofushi no oka no misasagi.
At some point Ōjin was made a guardian Kami of the Hata clan, and is now also deified as Hachiman Daimyōjin.
Outside of the Kiki, the reign of Emperor Kinmei (c. 509 – 571 AD) is the first for which contemporary historiography has been able to assign verifiable dates.
The conventionally accepted names and dates of the early Emperors were not confirmed as "traditional" though, until the reign of Emperor Kanmu between 737 and 806 AD.
While the historical existence of Emperor Ōjin is debated among historians, there is a general consensus that he was "probably real". If Ōjin was an actual historical figure then historians have proposed that he ruled later than attested. Dates of his actual reign have been proposed to be as early as 370 to 390 AD, to as late as the early 5th century AD. At least one Japanese historian has cast doubt on this theory though, by revising a supporting statement given in 1972. In this new narrative Louis Perez states: "only kings and emperors after the reign of Ojin... ...are seen as historical figures". In either case there is also no evidence to suggest that the title tennō was used during the time to which Ōjin's reign has been assigned. It is certainly possible that he was a chieftain or local clan leader, and that the polity he ruled would have only encompassed a small portion of modern-day Japan. The name Ōjin-tennō was more than likely assigned to him posthumously by later generations.
While the actual site of Ōjin's grave is not known, this regent is traditionally venerated at a kofun-type Imperial tomb in Osaka. The Imperial Household Agency designates this location as Ōjin's mausoleum, and is formally named Eega no Mofushi no oka no misasagi. At some point Ōjin was made a guardian Kami of the Hata clan, and is now also deified as Hachiman Daimyōjin. Outside of the Kiki, the reign of Emperor Kinmei (c. 509 – 571 AD) is the first for which contemporary historiography has been able to assign verifiable dates. The conventionally accepted names and dates of the early Emperors were not confirmed as "traditional" though, until the reign of Emperor Kanmu between 737 and 806 AD.
Emperor Ōjin