The Ōi River (Ōi-gawa) is a river in Shizuoka Prefecture, Japan. The Ōi River flows from the Akaishi Mountains, the branch of the Japanese Southern Alps which form the border between Shizuoka, Nagano and Yamanashi prefectures. These mountains, with peaks ranging from 2000–3000 meters, are characterized by heavy rainfall (up to 3000 mm per year) and deep V-shaped valleys. The river follows a generally southern course, with seven wide bends in its central region, before exiting into Suruga Bay in the Philippine Sea.
During the Edo period, the Tōkaidō developed as the major highway linking Edo with Kyoto, and daimyo from the western domains were forced to travel on a regular basis to Edo to attend to the Shogun in a system known as sankin kōtai. However, the Tokugawa shogunate prohibited the building of bridges over major rivers as a security measure, and in the case of the Ōi River, even ferryboats were forbidden. As depicted in contemporary ukiyoe prints by artists such as Hokusai, travelers crossed the river on bearers' shoulders or on horseback. In cases of bad weather or high waters, they were forced to stay several days (or even several weeks) beside the river at post stations such as Shimada-juku or Kanaya-juku.