Omi (臣) also styled (使主 おほみ Ohomi) was an ancient Japanese hereditary title denoting rank and political standing (a kabane) that, along with muraji, was reserved for the head of the most powerful clans during the Kofun period. The omi clans generally took their names from the geographic location from which they originated, such as the Soga (蘇我), the Katsuragi (葛城), the Heguri (平群), the Kose (巨勢), the Kasuga (春日) and the Izumo (出雲), thus making them regional chieftains in their own right. By tradition those who held the kabane of omi were considered branches of the imperial line (皇別氏族, kōbetsu shizoku), and they claimed that they were descendants of Emperor Kōgen, although there is no historical evidence to support this.
The most powerful omi added the prefix Ō (大/great) to omi referred to as Ōomi (大臣). Examples of Ōomi mentioned in the Nihon Shoki included Kazuraki no Tsubura (葛城円) during the reign of Emperor Richū, Heguri no Matori (平群馬鳥) during the reign of Emperors Yūryaku and Seinei, Kose no Ohito (許勢男人) during the reign of Emperor Keitai and the four generations of Sogas who dominated the title during the 6th and 7th centuries: Soga no Iname, Soga no Umako, Soga no Emishi and Soga no Iruka. This title denotes supremacy within the court, with titular power belonging to the Ōkimi (later denoted Emperor), whether or not he actually holds power. Of note to those not familiar with Japanese pronunciation, these same characters (大臣) are pronounced Daijin to refer to titles beyond 670 A.D. in Daijō-daijin, Sadaijin, Udaijin, Naidaijin, etc.
When the kabane system was reformed into the eight kabane system in 684 following a series of coup attempts, the powerful omi of the time were given the kabane of ason, which ranked second under the new system, and omi itself was dropped to sixth in rank.