Oni was a system of conferring a rank above a certain court rank to the descendants of high officials, according to the court ranks those high officials, the descendants' forefathers, had under the system of the Ritsuryo codes in Japan. It meant to receive a rank owing to their forefathers. The system was also referred to as "oni no sei."
It was enacted by Taiho Ritsuryo (Taiho Code). According to Senjoryo (the regulations for the recruitment and promotion of government officials), ranks were bestowed upon the descendants when they become over the age of 21, and those eligible for oni were children and grandchildren of Koshin (Emperor's family), the children of the emperor's great grandchildren, descendants of shoshin (retainers) who were higher than third rank, and children of those who were higher than the fifth rank. Oni also applies to kuni (order of merit) and zoui (conferral of a posthumous rank). Oni system was modeled on the Ritsuryo system of Tang (China), but compared to the Chinese system, its extent of qualification was limited, and the ranks bestowed upon were high. This is where the characteristic of Japanese aristocratic society that places emphasis on the social standing of a family may be seen.
Imperial family and shoo (princes without imperial proclamation)
Children of imperial princes => Jushiinoge (Junior Fourth Rank, Lower Grade)
Children of princes without imperial proclamation => Jugoinoge (Junior Fifth Rank, Lower Grade)
Legitimate children of the Emperor's great grandchildren=> Shorokuinojo (Senior Sixth Rank, Upper Grade)
(a rank will be declined for illegitimate children.)
Legitimate children of the first rank => Jugoinoge (Junior Fifth Rank, Lower Grade)
The ranks conferred will decline as it gets lower
Legitimate children of Jugoi (Junior Fifth Rank) => Juhachiinojo (Junior Eighth Rank, Upper Grade)
(A rank will be declined for illegitimate children, as well as for their grandchildren.)