I just found this thread and am inquiring as to any results. I too have ancestors from Japn with the surname Oba. My maternal grandmother is Kiyoko Oba. I believe i found pictures of her from the 1920's to 1930's. She may actually be about 7 years older than what we were always told.She had a marriage (possibly) in Japan before marrying my grandfather in the 1950's in America. She may have a daughter in Japan, Masako.
I have been searching for my kumon as well.