Anti-Nazi Austrians carved this “O5” into the facade of the church, in objection to Hitler’s renaming their country Ostmark (notice that the O doesn’t have the double dot, called an “umlaut,” over it.) Austria’s native name is Österreich. In German, OE can be used as a substitute for O with an umlaut, and E is the fifth letter of the alphabet. So by saying O5, the Austrians are reclaiming that damned umlaut and their country’s name.