Op-up happens due to oversold situation. Sometimes passengers miss their flight due to misconnect or dumped on to another flight due to mechanical or other operational reasons creating a situation where there are more pax than the seats. The result is no different what would have been attained if all he passengers had been overbooked on that flight. Essentially an op-up is due to oversell in almost 100% cases barring a few exceptions when a pax might be upgraded as a gesture of goodwill towards those who volunteer to give up a seat, once again due to oversell. In very rare cases you can get an op-up for reasons other than oversell.