oubliette

n (esp formerly) secret dungeon or underground prison with an entrance only by a trapdoor in the roof （尤指旧时）地下密牢（出入口在顶部）.

osteo-arthritis, osteopath, osteopathy, ostler, ostracism, ostracize, ostracize, ostracise, ostrich, OT, other, other from, other than, other-directed, other-worldly, others, otherwise, otiose, otter, ottoman, OU, ouch, ought, ought to, Ouija, ounce, our, ours, ourselves, oust, out, out of, out of breath, out of control, out of danger, out of date, out of debt, out of doors, out of favour, out of focus, out of order,