Oregonia is an unincorporated community in northwestern Washington Township, Warren County, Ohio, United States, on the east shore of the Little Miami River about five miles northeast of Lebanon and six miles south of Waynesville.
The first settlement there was around 1802, where there was a mill operated at various times by Nebo Grant, Ignatius Brown, and David Brown. Around 1820, it was known as Freeport. The Little Miami Railroad reached the village circa 1845. On February 8, 1846, a post office, called Oregon was opened, as there was another Freeport in Ohio. Sometime after the 1880s, the town assumed the name of Oregonia. It continues to have a post office, ZIP code 45054. The post office serves portions of Washington, Turtlecreek, Massie, and Wayne Townships.
Today, Oregonia is a stop on the Little Miami Bike Trail which follows the former train route along the Little Miami River. Bikers, hikers, and canoers often stop at the market or a restaurant along the trail which follows the river on the former train route. In October of each year, the "Devils Staircase" motorcycle hill climb is held on a hill just south of the main settlement.
In 2006, the largest Planned Unit Development in Warren County history, called San Mar Gale, was approved for future development. The new development will abut Oregonia on the east side of the river, and about half of its 3300+ acres will be served by the Oregonia post office.