Together with his father Masamitsu OYAMA, his older brother Asamasa OYAMA, his younger brother Munamasa NAGANUMA and Tomoie HATTA, on March 25, 1183, he defeated the allied forces of MINAMOTO no Yoshihiro (Senjo Saburo SHIDA) and Tadatsuna ASHIKAGA － who tried to attack and invade into Kamakura － at the Battle of Nogimiya.
寿永2年（1183年）2月23日、鎌倉への侵攻を図った源義広(志田三郎先生)と足利忠綱の連合軍を、八田知家と父小山政光、兄小山朝政、弟長沼宗政ら共に野木宮合戦で破る。