A road runs to the slope of the volcano, from where you can see Mt Fuji on a clear day. From here, you can hike 45 minutes to the 754m summit to peer into the still-steaming crater. It's another 45-minute hike around the rim of the crater, with stellar views 360 degrees around – it's fascinating to observe the path of previous lava flows. The Ō-shima Tourist Association can help with taxis and bus schedules.
There's great interpretive signage in English at various points of interest (and concrete shelters, just in case).