Oe (title or appellation) (大兄)

Oe was an appellation and title for powerful Imperial Princes, used in Wa (ancient Japan) from the first half of the 6th century to the middle of the 7th century.
It is considered that the holder of the title was a successor to the Ookimi of the Yamato Kingdom (it was also called as Amenoshita shiroshimesu ookimi, the emperor in the later ages.)

Summary

There is no contemporary sources that directly describe the meaning of 'Oe'. Therefore, the present historians are inducing the meaning of 'Oe' by comparing the Imperial Princes who held the title of Oe. Although the opinions are different from one another in details, it is generally considered that Oe was the title for the Imperial Prince who were most probably inherit the throne among a lot of the Imperial Princes.

In those days, the order of succession to the Amenoshita shiroshimesu ookimi passed through the ookimi's sons like the eldest son -> the second son -> … -> the youngest son (Kyodai Shokei), and if the youngest son died, the next successor would be the eldest son of the eldest son of ookimi. The eldest son of the eldest son of ookimi was called Oe. In those days, as the polygynous system was adopted, it seems that there were several Oe during the same period, which often led conflicts over the right of succession to the throne.
(On the contrary, there is a theory that insists Oe was limited to one person.)

Brief history of Oe

The first Oe appeared in 'Nihon shoki' was the eldest son of Emperor Keitai, Magari no Oe (later, Emperor Ankan), who was active during the first half of the 6th century.
As Emperor Ankan did not have a son, his next oldest brother Emperor Senka succeeded him; however, as Emperor Senka neither had a son, their youngest brother Emperor Kinmei succeeded Emperor Senka (There are other opinions that Emperor Kinmei defeated Emperor Ankan and Emperor Senka, and that there existed the Court of Emperor Kinmei and the Court of Emperors Ankan and Senka during that period.)

After that, there were several Oe such as Yatatamakatsu no Oe, who was a son of Emperor Kinmei; Oshisaka no hikohito no Oe, who was a son of Emperor Bidatsu who succeeded Emperor Kinmei; Tachibana no toyohi no Oe (later, Emperor Yomei), who was an older maternal half-brother of Emperor Bidatsu; Yamashiro no Oe, who was the eldest son of Shotoku Taishi, a son of Emperor Yomei and a regent for Empress Suiko; and Furuhito no Oe, who was the eldest son of Emperor Jomei, however, only a few of them succeeded to Amenoshita shiroshimesu ookimi. That means that the order of succession to the throne had not been clearly established in those days, and that the Imperial Princes held the title of Oe did not necessarily succeed to Amenoshita shiroshimesu ookimi. Instead, after the death of Empress Suiko, there was a conflict over the succession to the throne between the heir to Oe (Tamura no Miko, later Emperor Jomei) and Oe, who was the heir to the regent, (Yamashiro no Oe).

Naka no Oe (later, Emperor Tenchi) is supposed to be the last Oe. Ootomo no miko (Emperor Kobun) who succeeded Emperor Tenchi was no longer called Oe, and thereafter, the title Oe was not used. That means that the order of succession to the throne had supposedly been clearly established in those days. Probably, the established order was not to pass the throne to his brothers, but to his eldest son immediately after the emperor died. That might have been a reason that Ooama no miko (later Emperor Tenmu) rose in revolt (the Jinshin War), because Ootomono miko, the eldest son of Emperor Tenchi, succeeded to the throne and Ooama no miko failed to succeeded the throne.