A great-grandson of the famous scholar Ōe no Masafusa, he was born to Ōe no Koremitsu and adopted by Nakahara Hirosue but later returned to the Ōe 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 the Jōkyū War he insisted on making a sudden attack to Kyoto and contributed to the shogunate's overwhelming victory. He died after backing up Hōjō Yasutoki's succession. His fourth son founded the Mōri clan.