The teaching at Oxford (and Cambridge) is stellar and more Americans should consider it as an option. Getting onto an undergraduate programme is competitive but egalitarian: you need to show you are a top performer academically and then impress at an interview. Your competition at the application stage is (very roughly speaking) the smartest kid from every British school. Very few of them will get in; every course has many more applicants than places.
The Oxford University Entrance requirements, http://www.ox.ac.uk/admissions/undergraduate_courses/courses... , state the following:
"US qualifications: Successful candidates would typically have SAT Reasoning Test scores of at least 700 in Critical Reading, Mathematics and the Writing Paper, or ACT with a score of at least 32 out of 36. We would also expect Grade 5 in three or more Advanced Placement tests in appropriate subjects or SAT Subject Tests in three appropriate subjects at 700 or better."
- I have no idea what that all means, but for comparison, every single Brit applying will have straight A-grades in their A-levels, and selection is almost entirely based on the interview (and in some cases, extra advanced examinations). So take these entrance requirements with a pinch of salt; they're necessary but not sufficient.
All that said, the colleges have extremely wide lee-way in how they make their offers, so exceptional candidates with a slightly offbeat scholastic record might have a chance. Some cynics also note that foreign students may have an advantage because they bring in far more money. But all in all the academic requirements are quite high. To sum up, it's very hard to get in, but not because of prejudice or snobbery, just intense competition.