I'm confused about JD degrees and LLM degrees?

So I'm quite young and I'm nowhere near the age of applying to universities yet, but I'm certain that I want to become a lawyer (preferably either a cooperate lawyer of an IP lawyer). Anyway, I'm slightly confused at the moment as I was planning on (hopefully) studying law (jurisprudence) at Oxford for 3/4 years once I've completed my A-levels, then to go on to further education I want to go to Harvard to get a masters in Law- I'm confused as to if my previous degree from Oxford would allow me into this course or not? Also, as Oxford is obviously in the UK and Harvard obviously is in the US would it hurt my chance of being allowed on the course? Should I just try to get my undergraduate degree at Harvard and stay there for my masters? The reason I'm so intent on the US is that because I would love to work at NY-based law firm, so I should probably get my higher education in the US? Of course, I understand I'll have to take the Bar in NYC to be able to work there anyway, but studying it in the US surely would help me, would it not? Thank you x

First of all, to answer your question: a JD is the equivalent of an LLB in the UK (although it is held to a higher standard, since it is a postgraduate degree in the US). The degree that you are looking for is an LLM after you complete your BA in law at Oxford.

It would be possible to apply for an LLM afterwards, but note that it is an academic degree, meaning that it won't directly get you to your destination. However, if you want to go for the New York Bar, you would need to complete an LLM degree at an ABA-accredited school before taking your exams for the New York Bar.

As for studying in the US, remember that it would take you much longer and a lot more financial resources in order to fund your education. On top of the 4 years of undergraduate studies, you will have to compete with so many students that you would need excellent grades and a really good LSAT score in order to secure a spot at a T14 law school. You will have to complete 3 more years of law school before being considered for the Bar.

Doing your law degree in the UK would significantly speed up the process, although having a foreign qualification might put you at a disadvantage over other candidates with JD degrees (although if you supposedly got a degree from Oxford and a degree from Harvard afterwards, I think you've got a good chance, provided that your grades are good).