The name has its origin in Oyumi-jo Castle in Chiba County of Shimousa Province (the present whole area of both Oyumino, Midori Ward, Chiba City & Omi, Chuo Ward, Chiba City), where they established their base.
下総国千葉郡小弓城（現在の千葉市中央区_(千葉市)生実・同緑区_(千葉市)おゆみ野の一帯）に本拠地があったことから、その名がある。