Oo-yu is the largest of three communal bathhouses (sotoyu) in Bessho Onsen, Ueda City, Nagano Prefecture. It is known as Aoi-no-Yu (Hollyhock Bath) after the legend that the warrior Kiso Yoshinaka bathed here with his consort Aoi-no-Mae. Bessho Onsen boasts a 1,400-year history as the oldest hot spring in Shinshu, and the area is called the Kamakura of Shinshu for its abundance of historical temples and shrines. Oo-yu uses a blend of Source No. 4 and its own Oo-yu source, combining free-flowing natural spring water with circulation filtration. Source No. 4 was drilled from a depth of 350 meters, producing water at 52.5 degrees Celsius with an abundant flow of 1,300 liters per minute. The water quality is a simple sulfur spring (mildly alkaline), beloved as a beauty spring. Located in the heart of the Bessho Onsen district, it serves as a popular starting point for exploring the hot spring town. The waters are effective for neuralgia, muscle pain, joint pain, sensitivity to cold, fatigue recovery, and chronic skin conditions.