Nanbu clan
Ōkubo Tadatomo was a son of Ōkubo Noritaka, a 6000 "koku" "hatamoto" in the service of the Nanbu clan and descendant of Ōkubo Tadachika. Due to the early death of his father, Tadatomo was adopted by his brother Ōkubo Tadamoto, the 1st daimyō of Karatsu, whom he served as a page.