The interview at Oxford is NOT optional- nobody is accepted without an interview. Skype is a standard, frequently used option for people who can’t fly in. I’m not sure whether this is a typo:
“Also, do you think I should visit Cambridge for the interview? or take it in the US?”
or whether you are also considering Cambridge University? (you can only apply to one or the other). Either way, neither Cambridge nor Oxford interview in the US, and Cambridge does not do Skype interviews.
For math, as long as you can get the SAT to within shouting distance of 700 (I know somebody who got a place at Oxford in English with a 680 math), it won’t rule you out.
For Oxford, you will have to take the History Aptitude Test (HAT) and that can take a time to organize, especially if your school hasn’t done it before and there is no British or International School nearby. Be sure to start organizing that early, as the registration deadline is October and the test is in November.
If you are applying for History take as many APs / SATIIs as you can- US, Euro, World. Comp Gov only counts as 1/2 an AP, but it is actually helpful with some of the first year modules, and is a pretty easy self study. If you are applying for Hx + Eng, you will also want Lit & Lang, which are also easy self-studies.
Also, it’s not exactly a History (or History & English) major. It is a History (or History & English) degree- and that is all you study. First year you choose one British History class, one General History class, one Historical Methods class and an Optional Course (page 5 of this doc has a list of the options from a couple of years ago: http://www.history.ox.ac.uk/fileadmin/ohf/documents/History_at_Oxford_2012_FINAL.pdf). Compare that to the classes that you would take if you did History + English: http://www.history.ox.ac.uk/prospective/undergraduate/studying/joint-schools/history-and-english.html
and decide which one you would love more
Your Personal Statement (PS) - the essay- is where you make your case for why you are well suited to history (or history + english). To quote from an Oxford admissions podcast (find it on iTunes), “we want you to love our subject as much as we do”. One of the ones that you demonstrate your love for the subject is by the time you spend on it outside of the classroom- reading you do on your own, for example, or ECs that link to what you love about your subject. You will want to work on your PS over the summer, b/c it takes several goes to get one that is mature, thoughtful and concise (it’s a brutally short word limit).