Don't let the green appearance of the water here fool you, the water is crystal clear--the green comes from the tiles used the baths. This is one of the most complex, for lack of a better word, onsen that I have bathed in. It has a very cleansing quality to it while also feeling very medicinal to the point of being feeling unnatural. I doubt drinking this onsen would be OK given the trace amounts of arsenic and lead in the water. Arsenic, while fatal if ingested, is found in many of the world's most famous hot springs and good for you in small doses in a bath. While in the water it felt a little slippery, but the real magic of this onsen really became apparent after I dried myself off and put on my clothes. I felt great! So clean and refreshed.
The baths at Otomezuka are all built into odd shapes offering lots of little corners to inhabit, but they also don't have many good places to get in and out. Sadly, they are quite small given the popularity of this sento. The sauna is also very small. While hot and enjoyable it was a bit overpriced at ¥380 yen. I enjoy the small, odd shapes of the baths. It makes the experience more like an onsen playground, something common in the Yuasobi group of sento, but I can see how people used to the wider baths found in most supersento might dislike it. Masurao no Yu seems more packed in than the wider and open feeling Kagura no Yu side.
Otomezuka's three floor design is common among many sento in Kansai. The jet baths and some extra showers are placed on the mid-level between the changing room and the larger bathing area. Each side of this onsen has a different feel; the Masuro no Yu side being more eclectic with abstract tile work, and the Kagura no Yu side having a more sedated traditional Japanese feel. Architecturally speaking this building is a living relic of a Japan that has long since changed. Some of the building is a little worse for wear with its age showing through, but not to the point of distraction in the slightest.
This is a local place to the core. More so than the smaller Nada onsen, which I can see attracting many people from the newly built high rise apartment buildings nearby. Otomezuka seems to attract the people who have lived in this neighborhood for years. Being a foreigner I always get a few double takes here from the local costumers, but I've always felt invited and relaxed. There is also a food counter here with a few great deals. Beer and a snack for ¥500. Also, CHEESE FRIES! Yes, cheese fries and a beer after a soak. Could life get any better?
Update February 2020: I have always been on the left side of the bath recently so it was fun to get the right side this time. I forgot how cool the outdoor natural hot spring and cold water bath on this side was. Neat, 80's squares and angles are a good counterpoint to the natural stone of the other side. I went to here as my last bath to complete the Kobe sento stamp rally and was happy that I did.