Okinaga supported the Hosokawa Clan by serving four different heads of the clan (Tadaoki HOSOKAWA, Tadatoshi, Mitsunao HOSOKAWA and Tsunatoshi HOSOKAWA) in the years since he first went into battle at the age of 19 in 1600 until his death at the age of 80.
興長は慶長5年（1600）の19歳での初陣以来80歳で亡くなるまで細川忠興、忠利、細川光尚、細川綱利の4代の主君に仕え、細川氏を支えた。