I've only been QP for 2 years and SG for a couple years before that, but received a few op-ops. For QF metal, just the one, where I had booked into Premium Economy on JNB-SYD, paid flexible fare. I was op-up'd to business - it was a weekend during school holidays as mentioned by others. I think I was only gold then but I suspect I was picked due to the amount I had paid for the seat.
The best op-op's have come from OWE status on partners. I've received one on AA (domestic Y-F), two on Cathay Dragon (short haul Y to J), and the best one which was on UL (CMB-MEL), from Y to J (long-haul full flat bed). The latter was a week or so after the Sri Lankan bombings so I suspect business travel had tanked yet Y was oversold with locals/expats trying to get away. I am unhappy about the tiny number of SC's you sometimes get on OW partners (and am going to have trouble retaining WP as a result), but the op-up's have been a good consolation.