This code ‘O5’ is placed at the right side of the main entrance of St Stephen’s Cathedral. It was the code of an Austrian resistance group during World War II. It stands for Ö (Österreich) built by O and the 5th letter of the alphabet (= OE).
Originally the code was just painted but after World War II it has been engraved and covered by a transparent plate.
- Widerstandsgruppe O5 (Wikipedia)