Keisan Osen entered the priesthood at an early age and underwent religious training at the Sokoku-ji in Kyoto. To avoid the troubles of the Onin War, he took refuge in the province of Omi, but returned to Kyoto in 1472 and resided at the Sokoku-ji and later the Nanzen-ji. At this time Zen monks generally won fame either by becoming outstanding scholars or by acting as political advisers.
Osen Keisan followed the former course, and was allowed to accompany the Japanese embassies to Korea in 1472 and 1474 and the embassy to China in 1475. So great was the respect accorded his ability in Chinese studies that he was selected to compose the official letters that were sent to the courts of China and Korea. A collection of his verse and prose in Chinese, the Keikashu, has been handed down and holds a place of importance in the Gozati bungaku, the body of poetry and prose works written in Chinese by the Zen monks of the late Kamakura and Muromachi periods.