Approximately 15 miles (24 km) south of Elkton, the highway departs from the river, and heads east-southeast until it reaches the city of Sutherlin.

It heads due east out of Roseburg, ascending into the Cascade mountains, and turning northeast.

Just south of there is an intersection with Oregon Route 230, which bypass Crater Lake National Park to the west.

OR 138 continues east from the entrance, descending into Central Oregon, and terminates at an intersection with U.S. Route 97 at Diamond Lake Junction, south of the town of Chemult.

The portion of the highway between Roseburg and Diamond Lake is part of the Rogue-Umpqua Scenic Byway.