An oyster hood us a kind of hood, so called because its shape is supposed to resemble an oyster.
It consists of two hoods, one inside the other, joined at the neck. The inner hood is a normal one; the outer hood zips up with the zip vertical across the face. Thus when the zip is open the inner hood is visible. If this inner hood has eye holes, the wearer can see with the zip open but not when it is closed.