Oe no Hiromoto (大江 広元 Ōe no Hiromoto; 1148-1225) was a vassal of the Kamakura shogunate in Japan of aristocratic origin. He contributed to establishing the structure of the shogunate.
He was a great-grandson of the famous scholar Oe no Masafusa. He was born to Oe no Koremitsu. He was adopted by Nakahara Hirosue but later returned to the Oe family in 1216. There is another theory that Hiromoto was born to Fujiwara no Mitsuyoshi. As a minor noble, he originally served at the Imperial Court in Kyoto.
In 1184 he was invited to Kamakura by Minamoto no Yoritomo, who later founded the Kamakura shougnate. He became the first head (別当 bettō) of the new Kumonjo (Board of Public Documents) in the same year and then of the Mandokoro (Administrative Board) in 1191. On Hiromoto's advice, Yoritomo appointed Jito and Shugo in 1185, which helped to strengthen the shogunal control over provinces. In 1190 he followed Yoritomo to Kyoto and remained there to negotiate with the imperial court until 1192.
After Yoritomo's death, won his widow Hojo Masako's trust and assisted in Hojo clan's seizure of power. He was involved in several important event in the shogunate. In 1199 real power was moved from second shogun Minamoto no Yoriie to the council of influential gokenin. In 1203 the shogun got arrested and his supporter Hiki Yoshikazu. Hiromoto also helped the Hojo clan crush its rivals including Hatakeyama Shigetada, Hiraga Asamasa and Wada Yoshimori.
In the Jokyu Incident he insisted on making a sudden attack to Kyoto and contributed to shogunate's overwhelming victory. He died after backing up Hojo Yasutoki's succession. His fourth son founded the Mori clan.