We all make the educated guess that China is in the same state as the US. As previous posts have said, the intro to FO1 "In two brief hours most of the planet was reduced to cinders."
I think its safe to say that when every country that has Nuclear arms fires them at each other, not much is gonna be anything but a desolate wasteland.
I second that. 200 years have passed without anyone hearing a thing from the rest of the world. This is the clearest indicator anyone could point to that the rest of the world went as back to the stoneage as the USA did.
Otherwise someone just would have shown up in the USA at some point. But obviously there are no real shipping nor air traffic capabilities left in the rest of the world either.
China definitely would have come back. And if not China then Europe or Australia. Nobody would leave that oil there if he still had the chance to get it.
Once the USA was basically gone , everything valuable that was still there belonged to everyone who had the means to go there and just take it for himself. But as far as we know nobody ever came.
China would have secured the Canadian oilfields and ensured that the USA does not pose any further threat. For instance by going in and securing any military sites like airfields, missile silos, weapons depots, bases etc, basically anything that may have survived the nuclear strikes or was not completely flatened so that they could at least theoretically be made ready again and used for continuing that war.
But as this obviously never happened the reason must be that China could not do anything like that because , as a country , the PRC did not survive the year 2077 just like the USA.
Europe would not sit around and let those oil reserves catch dust either. It would went over and secure them for itself once the USA is gone. Australia would be another prime candidate for getting whatever nobody can refuse them and become the new king of the hill. Just out of necessity.