Oyama Tomomasa (小山 朝政, 1155 – May 15, 1238) was a Japanese samurai lord and gokenin of the late Heian and early Kamakura period.

[1][2] Tomomasa was born in 1155, in Shimotsuke Province, the eldest son of his father, Oyama Masamitsu,[1][2][3] and his mother, Samukawa-no-ama.

[2] He distinguished himself at the Battle of Noginomiya,[1] defeating Minamoto no Yoshihiro,[3] who had attacked Shimotsuke with Ashikaga Tadatsuna,[2] in 1183.

[3] An influential gokenin during the founding of the Kamakura shogunate, he joined Minamoto no Yoritomo in the Battle of Ōshū in 1189.

[6] He accompanied Yoritomo during his entry to Kyoto in 1190, and was appointed Lieutenant of the Right Division of Outer Palace Guards.