I had an experience with OS on a sunday evening flight VIE-ZRH. They used a 737-800 and at the Business Class Check-in the lady told me they were overbooked (5 pax in eco - i was also travelling in eco) and they need VDBs. I volunteered and also offered to be rerouted via MUC/FRA or put on a 1-hour earlier LX-flight, which they denied, because they wanted to wait for the actual situation (not all booked pax really arrive, or some might miss the connection).

So, I went to the Business Class lounge and waited for my flight. When leaving the Lounge the helpful agent informed me, that not all pax checked-in, so I can take my flight. I walked straight to the gate, where I boarded as one of the last passengers and to my surprise, the gate agent handed me a C-Class Boarding pass and said thank you for offering to VDB.