Ōseki Masunori

Viscount Ōseki Masunori (大関 増勤) was a Japanese samurai of the late Edo period who served as daimyo of the Kurohane Domain (Shimotsuke Province). Succeeded to family headship in 1868, amidst the turmoil of the Boshin War. His domain was involved in the Battle of Utsunomiya Castle.

After a brief period in the 1870s when he studied in the United States, Masunori returned to Japan, and was made a viscount (shishaku 子爵).

He was a recipient of the Order of the Sacred Treasures.