The Octagon Barn, Otter Township is a historic building located near Milo in rural Warren County, Iowa, United States. It was built in 1900 as a horse and dairy barn. The octagon-shaped building measures in diameter. The structure features a tall center section with a winged shed around it. It is covered in red horizontal siding and is topped by a sectional conical roof. It has been listed on the National Register of Historic Places since 1986.