The chemical compound osmium tetroxide (OsO4) is an oxide of the element osmium, in which it attains its highest oxidation number of 8.
It is a liquid at room temperature and is readily formed when osmium is exposed to air. In fact it explains the name of the element, derived from a Greek word for stench. The oxide is a powerful staining agent in regular use in transmission electron microscopy (TEM). It provides contrast to the image. It also stains the human cornea, which can lead to blindness if proper safety precautions are not observed.
In organic synthesis it is sometimes used to oxidise alkenes to the dialcohols. One catalytic reaction with osmium tetroxide is the Sharpless bishydroxylation named after K. Berry Sharpless.
