Oscausi is the incantation for a spell that causes a person’s mouth to be “sealed shut as though she never had one” making the orifice disappear (CG67).
References from the canon
This spell was used by Leta Lestrange on a girl from Gryffindor when they were both in their third year at Hogwarts. The Gryffindor girl was speaking ill of Lestrange behind her back until Lestrange emerged from hiding nearby, and muted the Gryffindor girl with this spell (CG67).
Possibly derived from the Latin of os, for "mouth", and clausi Latin for "I shut".