Ondo is one of the subjects of Daigaku-ryo, an educational institution under the system of centralized administration established under the Ritsuryo Legal Codes in Japan, to learn the Han pronunciations of Chinese characters in order to allow the reading aloud of the scriptures of Confucianism with the original pronunciations.
According to the "Nihon Shoki" (The Chronicles of Japan), its history goes back to a time when Emperor Jito invited and appointed Shugen ZOKU and Kokaku SATSU to become Professors of Ondo in 691. After the Ritsuryo Legal Codes were established, the Professors of Ondo were positioned within the Daigaku-ryo system, but because their role was to teach the reading aloud of the scriptures during the first course of Confucianism to the students of the main course (later known as Myogyodo, the Confucian department of Daigaku-ryo), their position was only deemed to be supplementary and it was not mentioned within the regulations that they may have their own students like Sando (Mathematics) and Shodo (Calligraphy) Departments.
However, the increasing influence from the Chinese Tang culture of the early Heian period, Emperor Kammu issued a decree to encourage the Han pronunciations of the Chinese characters on January 11, 793, which in effect forced the students learn it ("Nihongiryaku" (Abstract of Japanese History)), and on March 10, 798, students were prohibited from learning Goon, the existing pronunciations from the Southern Dynasty in China were prohibited and were instructed to learn the Han pronunciations of the Chinese characters ("Koninkyakusho" (Regulations in Konin period)). Similar laws were passed which applied to monks and Professors of Ondo were sent in to perform examinations of the monks, and further, on June 15, 793, the law stated that monks must master the Han pronunciation in order to enter the Buddhist priesthood ("Nihongiryaku," "Ruijukokushi" (Classified History of Japan)), and then on June 3, 801, the Examination of Han Pronunciations of Chinese Characters was imposed on the monks entering the Buddhist priesthood ("Nihongiryaku"). During Emperor Saga's reign, an ordinance was issued on May 10, 817 to establish the 'Kangosei' (students of Chinese), which composed of six students including four Onsei and Hakucho.
But after the Japanese envoy to the Chinese Tang Dynasty ceased in 839, there were hardly any need for the Han pronunciations of Chinese characters in Japan, and as such, Ondo's popularity declined rapidly. It was most likely absorbed by Myogyodo even before Calligraphy and maintained a temporary place there until the Nakahara clan and the Kiyohara clan got appointments to Myogyodo and were later able to dominate Myogyodo.