An owl is associated with Halloween in multiple ways: as a familiar of witches, warlocks and other magical beings, for their eerie night calls, for flying at night and in general as a creature of the night and hunters.
Owls may be used as imagery in decorating. Stuffed owls are useful as accessories to the appropriate costumes.
Owls in many places were thought of omens of death. If someone saw an owl or it screeched outside their bedroom window they believed they would soon die.