US only has two classes of service -- F/Y domestic/north america, C/Y trans-atlantic (A330 trans-atl has old F/C/Y seating, but only two classes of actual service).

As for op-ups, they're extremely rare domestically/north america because of the unlimited upgrades for status flyers. (They'd have to have an oversold flight in Y without enough status members to fill F.) They might come up a few times on trans-atlantic flights but I've never had one happen on any of my flights. I'd guess that they would be for US status members, but I have no idea.

That's all I can help with.