The Onii-chan Onsen is a hot spring (onsen) bathhouse built by Molster. The bathhouse, as the name suggests, is of Japanese style, with a changing room and a cold bath room as well as the hot spring pool itself. The building also has lodgings on the upper floor.
The bathhouse was located northwest of Cowville before Gamingmaster000 tore down Cowville to make way for Morioh, annexing the bathhouse in the process.
The changing room. Yup, you strip down everything there in the open.
The coldwater bathroom. Everyone must wash here before entering the hot spring pool. And DON'T drop the soap!