Ooser (“Rhymes with bosser, not boozer,” I always tell people) is a term from what Sybil Leek would call the Language of Witchcraft. It denotes a carved and horned wooden head-mask of the God of Witches.
It's a dialectal word, of unclear etymology. Doreen Valiente suggests an origin from ós, the Old English cognate of Old Norse áss, “god,” better known to English-speakers in its plural form aesir. An ooser, then, would be a “god-er,” which, since it bears the god and is worn by his personifier at the sabbat, makes sound theological (if not etymological) sense.
The famous and mysterious Dorset Ooser is the best-known example. Also known, from its bull-horns, as the “Yule Bull,” it frightened generations of Dorset children until it was stolen from its hereditary keeper in 1897 and never recovered. Old Craft scuttlebutt would have it that it was “took” to get it out of cowan hands, and that it has since remained in ongoing, if private, use among witch-folk to this day.
Well, so they say. In its own way, it's even a true story.