St Andrews is popular in the US, particularly at prep schools in the northeast, given the ever increasing selectivity of the Ivies and similar private universities. In fact, behind the UK (~56% of St Andrews students), the US is by far the best represented country at the university (14.7%), followed by Germany (3.3%), China (3.2%), and Canada (2.2%).
The percentage of students from North America is far smaller at Oxford – only 200 out of 12,000 undergrads (1.7%).
Oxford is a pretty special place, for sure. I spent several months of last year doing research at Oxford and didn’t want to leave. If I could do college all over again and had the funds, I’d choose it over any American school in a heartbeat.