WoO is an acronym/abbreviation, derived from the German musical catalog phrase Werke ohne Opuszahl (English: "Works without opus number"). WoO is a catalogue prepared in 1955 by Hans Halm and Georg Kinsky, listing all of the compositions of Ludwig van Beethoven that were not originally published with an opus number, or survived only as fragments.
The abbreviation is also used sometime to refer to works without opus by other composers.