Tries from Williams, Phillips and a superb team-try finished off by Byrne emphasised the huge gulf in class as the Ospreys ran out comfortable winners.
The Foxes sealed the win when a superbteam move saw the ball break to King, who finished smartly from 12 yards as the Owls slipped into the bottom three.
But despite their underdog status, the United States, backed by a vociferous crowd, produced a superbteam performance and they never looked back after leading from the first match of the first session on Friday.