Ohio State Route 198 is a short route starting in Wapakoneta and running north-northwest for 12 miles. The southern end is an interchange with US Route 33, where Ohio Route 198, Ohio Route 67, and Ohio Route 501 all start. The three routes run north to a central junction in Wapakoneta, where Ohio Route 67 splits off toward the east. Soon after, Ohio Route 501 splits off on the northwest side of town, leaving Ohio Route 198 to continue toward the north-northwest. The northern end of Ohio Route 198, at Ohio Route 117, is west of Lima, but it's east of Spencerville.