The only likely situation IME when a plane is mostly empty and an OpUp happens usually has to do with Premium Economy. In fact; on a flight from LHR to SFO earlier this year - J was half empty, Y was pretty empty, but they managed to sell most of the WTP+ seats prior to the day of and then it looked like they sold another 7-8 at check-in to customers. This caused a -6 or -7 oversell situation in premium economy and several (including yours truly) got op-upped to Club. Club was so empty I had a whole middle section as a cocoon and they were spreading out passengers so they weren't next to each other for take off. If your flight is W9 as well though, highly unlikely for an op up