On QF, having done OLCI makes no difference to whether you'll receive an op-up or not as Altea will look at the PCV of the pax not their checkin status.
Not sure how VA do their op-ups and if their checkin system can display a list of pax according to their PCV. I suspect it's more of a manual process where VA staff would just choose somebody rather than the system recommending someone.
I'd say that was possibly the case pre-Altea however generally speaking for international flights op-ups would be processed by yield management several hours prior to departure based on the PCV so IMHO it would be the exception rather than the rule that a gate agent had any say in the matter.
It may need to be done perhaps if there was a last minute issue eg a seat that was broken or some other last minute unforeseen reason however the authority would need to come from higher up as gate agents don't have authority to upgrade people. The last thing you'd want is someone getting an op-up then telling all and sundry onboard they've been upgraded.
A gate agent/CSA can reprint a new boarding pass in a higher cabin (when a supervisor or yield management have been the ones to action the op-up) which can sometimes be misinterpreted by pax thinking that the person handing them their new boarding pass has been responsible for actioning it.
When we were upgraded at the gate on Emirates I got the impression that decision had been made long before the flight gate opened up. The guy in front of us asked at the gate about paying for an upgrade but the host said the flight was full. We thought we'd had no chance. Thankful to be proven so very wrong.
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