Ōkimi (大王, also read as Daiō), or Ame no shita Siroshimesu Ōkimi (治天下大王, Chi Tenka Daiō), was the title of the head of the Yamato Kingship, or the monarch title of Wakoku (Old Japan). This term was used from the Kofun period through the Asuka period in ancient Japan.

The title 大王 (Ōkimi or Daiō), which is an honorific title for the head (king) of the Yamato Kingship, was established around the 5th century and was used until the 680s. It was established when the compilation of the Asuka Kiyomihara Code started. There are several theories upon whether the title holder in the early period is a king of the unified kingship or not. Ōkimi in Japanese reading is created by adding the prefix ō or oho which indicates greatness and particular nobleness, to the title "kimi" (lord), which indicates a master or nobleman. Another theory states that Ōkimi is only an honorific form of kimi, a title with Japanese origins, while Daiō is based on a king title which originated from China; the title of kango (Chinese word). There are several instances of the use of Ōkimi, which is understood as a courtesy title of Emperor or royal family.

The kanji title "王" (Ō, Wang) originally designated a Master of Chūgen (中原, Zhongyuan) in Inner China. In the Zhou dynasty period, 王 (Wang) was the title of the sole Son of Heaven who rules the Tianxia. However, some great powers in the region of the Yangtze civilization did not want to stand in subordinate positions of the nations of the Yellow River civilization in North China, such as Chu, Wu and Yue. Some of their monarchs titled themselves Wang.

When China entered into the Warring States period, the monarchs of the great nations among the states of North China who were originally subjects of the Zhou King, but achieved territorial statehood, called themselves sole Wang of the Tianxia in place of the Zhou King. Thus, there were numerous claims to the throne in mainland China. Thereafter, Ying Zheng (嬴政), the King of Qin (Emperor Shi Huang), who unified China for the first time in 221 BC, adopted the title "Emperor" (皇帝, Huángdì) instead of title "King", which had been degraded. The King title Wang became the title granted to subjects of the Emperor, or that assigned to heads of neighboring states who recognized the authority of the Qin Emperor as Master of Tianxia, with a connotation of a subordinate rank. The latter usage was established thereafter. The monarch of Xiongnu stood on even ground with the Emperor of Qin, therefore his title was Chanyu, not Wang.

The first appearance of King title related to old Japan is 漢委奴国王 (Kan no Wa no Na no kokuō, King of Na in Wa of Han) engraved on the gold seal which was bestowed on the king of Nakoku by the emperor Guangwu of Han in 57 AD.

The word Wakoku ō (King of Wa) appears in the article dated to the first year of Eisho (107 AD), in the record of Emperor An in the Book of the Later Han. The full title, as written in the record of emperor An, is: "Suishō, the king of Wa, and other". If "Wakoku ō" refers to the King of Wakoku as a head of the states union, other than a head of small regional state, this description shows the establishment of the Wakoku.

In addition, Himiko (c. 180 AD–c. 247 AD) was authorised as the unified Queen of Wakoku (whose capital was Yamatai koku) by the Wei dynasty. There is a theory that the government that existed during Himiko's rule was an early form of the Yamato Kingship, though this has been disputed.

The kanji letters of the title 大王 (ōkimi, great king) was first appeared in the inscription of the iron sword unearthed from Inariyama kofun, Saitama prefecture.