Would it be okay to reapply to a Ph.D. program after getting a Master's Degree elsewhere?
I think you should ask the three schools in the U.S. that offered you a spot.  But don't tell them the reason you shared with us for why you want to go to Oxford for a master's.  Just say it's an exciting opportunity.  

Ask whether you can defer your admission for a year or whether you should reapply.  If they say you should reapply, then do so.