The letters WoO, an abbreviation for the German "Werk ohne Opuszahl" ("work without opus number"), followed by a number are used to uniquely identify pieces of music by certain composers which do not have opus numbers. They are particularly used in reference to works by Ludwig van Beethoven – WoO numbers for his works were assigned by Georg Kinsky and Hans Halm in their 1955 catalogue of his works ("Für Elise", for example, is WoO 59).

For a list of WoO numbers, see List of works by Beethoven.