His family line could be traced back to the Oyama clan that was founded by FUJIWARA no Hidesato who was a Chinju-fu shogun (Commander-in-Chief of the Defense of the North); Tomomitsu's father, Masamitsu OYAMA, was the head of local ruling family in Oyama, Shimotuke Province (present day Oyama City, Tochigi Prefecture).
家系は鎮守府将軍藤原秀郷を祖とする小山氏の流れであり、父は下野国小山の豪族・小山政光。