Flying SFO-JFK on Friday.
I stood by on an earlier flight using OLCI.

My confirmed flight was completely full, in all classes, probably overbooked, too. Needless to say my upgrade was waitlisted.
When standing by, I looked for a flight with the most C seats so that I could maximize my already very slim chance of being upgraded.
When I got to the gate for the standby flight, I was pleasantly surprised by the GA telling me "You are on a later flight, right? Okay, I am going to put you in business."
I wasn't sure whether she had upgraded me using my instruments (but then she would have said more, like how would you like to pay for the upgrade, etc.) but when I looked at the boarding pass she handed me, (not the stub) it said booking class:W. So I guess it's an op-up. Otherwise it would have said NC, right?

They probably would have had to take VDB's for the flight I was booked on, and economy on the standby flight seemed to be full, so it made financial sense for them to do this, but does United often op-up standby passengers?

Anyway, three op-up's on p.s. in my past 5 roundtrips, the stars must really be aligned for me!