Ommyodo, which owes its origin to ancient Chinese natural philosophical theory and Inyo-gogyo-shiso theory (the way of Yin and Yang, combined with the Taoist theory of the five elements), is a system of natural science and the art of spell-casting that uniquely developed in Japan. The Chinese characters for Ommyodo can be read "Inyodo" as well. People who practise Ommyodo are called Ommyo-ji (diviners or sorcerers) and a group of Ommyo-ji is again called Ommyodo.
At one time, researchers thought that the philosophy of the Yin and Yang introduced into Japan developed into Ommyodo. In recent years, however, it is thought that Ommyodo developed as follows; the Ommyo-gogyo-setsu theory, that combined the theory of Ommyo (the Yin and Yang), which considers that the universe has been derived from the Yin and Yang, and the theory of Gogyo-shiso, which considers that the universe consists of the five elements of wood, fire, earth, metal, and water, developed uniquely in Japan into a practical fortune telling technique that could serve human needs based upon the auspicious and inauspicious signs obtained by observing the Yin and Yang in Nature and changes in the five elements, while being affected by Shinto, Taoism, and Buddhism.
When the Ommyo-gogyo-setsu theory was introduced to Japan together with Buddhism and Confucianism during the 5th and 6th centuries, it combined the studies and art of divination that involve the observations of Nature, including astronomy, Rekisu (a study to establish a calendar by calculating the solar and lunar movements), time science, Iching (Book of Changes), and became adopted in Japanese society as fortune telling techniques that could serve human needs based on the auspicious and inauspicious signs identified in Nature. Although initially, these techniques were practised by priests from overseas, especially from China and Korea, who had settled in Japan and were proficient at reading and writing Kanbun (Chinese classical literature), the later 7th century saw the appearance of Ommyo-ji as there was a need for laymen with the techniques, not priests, to serve the Imperial court.
From the later 7th century to the early 8th century, when the Ritsuryo system (legal codes of the Nara and Heian eras based on Chinese models) were put into place, the divination techniques of Ommyo (the Yin and Yang) were transferred to the Bureau of Divination under Ministry of Central Affairs. The Bureau of Divination was made up of divisions dealing with Ommyodo, Tenmondo (the art of divination by observing the movements of the sun, moon, stars and planets, and the weather), and Rekido (masters of the almanac) these three divisions performed divination, observed the movement of the sun, moon, and stars, and created a calendar. The Ritsuryo system, while prohibiting priests from practicing divination and observing the movements of the sun, moon, and planets, provided opportunities for Ommyo-ji to exclusively conduct state affairs.
From the Heian period onwards, when the Ritsuryo system declined and the Fujiwara clan gained power, Ommyodo, showed people how to avoid natural disasters through the art of divination and spell-casting and became guidelines that could affect the private life of the Emperor and court nobles, in contrast to Goryo-shinko (an off-shoot of Shintoism, its purpose to quell angry spirits of individuals who have been done injustices), which had become increasingly popular and more ritual in the imperial court. Ommyodo spread from the palace into Japanese society while becoming more generalized; thanks to the efforts by bonze Ommyo-ji, priests who practiced Ommyodo, it permeated deeply into the populace and developed more uniquely in Japan.
Japanese Ommyodo developed in a unique way under the reciprocal influence of Shinto while adopting the various arts and skills from Taoism, introduced into Japan together with Ommyodo; they include Katatagae (practice of reaching a destination taking a different direction than going directly from one's house by putting up somewhere the night before), Monoimi (fasting, abstinence, and confinement to one's house on an unlucky day), Henbai (special footsteps in the art of Japanese public entertainment, believed to have magical powers), Taizanfukunnsai (a spirit exchange ritual), Fusuisetu, and Jukondo (an art of medicine). From the end of the 8th century, Ommyodo came under the influence of Juho (a way of practicing self discipline by uttering mantras) from esoteric Buddhism, Sukuyo-do (astrology) and other arts of divination, which had been introduced into Japan together with esoteric Buddhism.
The 10th century saw the appearance of Tadayuki KAMO and Yasunori KAMO, a father and a son who mastered all Ommydo, Tenmondo, and Rekido, and ABE no Seimei, one of their disciples and a superb Ommyodo diviner, who was fully trusted by the imperial court. Tadayuki and Yasunori taught Seimei Tenmondo and Koei, Yasunori's son, Rekido, which led Ommyodo to completely encompass Tenmondo and Rekido from the end of the Heian period through the Middle Ages, allowing the Abe and Kamo clans to dominate the world of Ommyodo as the two Soke (originators).
From the end of the Heian period onwards, the Abe clan produced the masters of Ommyodo, one after another, helping the clan, who had been low-ranking nobles, get promoted to the level of court nobles. During the Middle Ages, the Abe clan transferred the post of Ommyo-no kami (the head of the Ommyoryo (Bureau of Yin and Yang)) by succession while the Kamo clan, who held the post of Ommyo-no suke (deputy minister of Ommyoryo), played a subservient role. During the Sengoku period, the Kadenokoji family, the head family of the Kamo clan, ceased to exist, causing their dominance over Rekido to be transferred to the Abe clan, while the Tsuchimikado family, the head of the Abe clan, gradually declined as wars continued. On the other hand, during the Muromachi period, Ommyodo permeated more deeply among common people, causing private Ommyo-ji to flourish as fortune-tellers and shamans.
When the Baku-han system (the feudal system characteristic of the Shogunate) was put in place, the Edo bakufu (the Japanese feudal government headed by a shogun) tried to control Ommyo-ji across the country by rebuilding the Tsuchimikado family and the Koutokui family, branch families of the Kamo clan. Over time, the Tsuchimikado family outstripped the Koutokui family, and by the end of the 17th century, they achieved a dominant position in Ommyodo across Japan by obtaining the right to grant Ommyo-ji licenses. Although Ommyodo had lost its political clout by the beginning of the Edo period, it became widespread and permeated deeply into Japanese society as folk beliefs; that people used to seek for an auspicious date or direction for special occasions.
In 1872 after Meiji Restoration, the Meiji government abolished Ommyodo as superstition. Except for Tensha Tsuchimikado Shinto Honcho (a school of Shinto and Ommyodo, based in Fukui Prefecture) started by the Tsuchimikado family, and the Izanagi ryu school, which has been handed down through the generations in Mononobe Village (the current Kami City), Kochi Prefecture, Ommyodo left its traces only in a calendar, but its impact on Shinto and new religions still lives on.