Opup is a system to upgrade platinums (opup-op) and golds (opup-og) no matter what inventory is like in coach. There can be 100 open seats in coach and up to 5 plat/golds will be upgraded. It's all automated and it done if the agent has time. This is formally the opup with NW.

Opup's do not happen due to oversold situations, those are over sale upgrades. I see why they are confused, but they are two different things with NW.