Graduate school in America after Oxford (Physics)

(Hi guys, it's my first time posting, so I apologize in advance if I make any mistakes)

I've been very curious about graduate studies in America after graduating Oxford (physics, to be specific).
Some say that there is a "barrier"; i.e., it is harder for Oxford graduates to enroll into decent American graduate schools (as opposed to American college graduates enrolling into American graduate school). Is this true?
Although the exact statistics (e.g. In year ****, X number of Oxford physics graduates went to Y graduate school...) would be impossible to acquire, I would like to have a sense of what actually goes on.

Merci d'avance!

P.S. I have yet to start my undergraduate studies; however, this question is a rather important factor in my university decision.

If you didnt start yet, that means you havent even got accepted yet ....

You would have a good chance, if you have decent grades and some good summer research experiences.