San-ei Oseki was born in Shonai District (Yamagata Prefecture), Japan in 1792.
He studied under German physician Jonkheer Philipp Franz Siebold in Nagasaki.
He was reputed as a master of Dutch medicine and language. He served the lord of the Kishiwada Clan (Wakayama Prefecture ) and became his physician in ordinary. Afterward he was attached to the Shogunate astronomical observatory.