Ohio State Route 521 is a short route in Delaware County, in central Ohio. The route actually begins in the City of Delaware, and it connects to areas in the northeastern part of the county. As US Route 36 and Ohio Route 37 head eastward out of Delaware, on the same alignment, they meet the southwestern end of Ohio Route 521. Ohio Route 521 runs northeast to the tiny town of Kilbourne; then, it heads easterly from there to cross over Interstate 71 with no interchange. The end goal of Ohio Route 521 is to cross Ohio Route 61 and reach Olive Green, an unincorporated hamlet where it ends at Ohio Route 656.