We stayed at Oohira hotel in October to use it as a base for hiking. It is perfectly located, near the top end of Zao Onsen, right next to the base of the mountain and the big Zao outdoor onsen. We found it with no issues, and the hotel will pick you up from the bus station if you call in advance. The main thing to stand out for us at Oohira was the staff, who were really excellent throughout. They even spoke some English which was surprising and welcome! The hotel building itself is a little 1970's and could do with a lick of paint in the corridors, but it was perfectly clean. The onsen was really nice, hot, steamy and free shampoo and shower gel on offer. The food was excellent too with lots of, somewhat unrecognisable, food to try and be brave with. Though there was good shabu shabu, beef sukiyaki and really standout sashimi. Overall a pretty safe bet for an authentic Japanese ryokan experience!