"Over the Willamina-Sheridan Highway from its junction with the Salmon River Highway, OR18, at Wallace Bridge, northerly and easterly via Willamina and Sheridan to its junction with the Salmon River Highway, OR18, east of Sheridan."
~ ODOT, Descriptions of US and Oregon Routes, March 2007
OR-18 Business previously existed as a part of OR-18 that went through Willamina and Sheridan from OR-18's beginnings in 1932 all the way until 1957, when a bypass was built both of those two cities. The former route was thus renamed OR-18 Business, and the highway was kept in the Oregon Highway System as the Willamina-Sheridan Highway #157 (previously it had been designated as a part of the Salmon River Highway #39).
