The letter should be sent according to the college or department to which the person you sent it belongs. The zip code of each college is different. The geographical location of Oxford University is: England-Oxfordshire-Oxford City. However, the sending and receiving of mails are independent of their respective colleges, so you can't send the words "Oxford University" or "Oxford University" directly, because no building can be called "Oxford University", it is a whole composed of several colleges and departments.

For example, if I am from Baliol, you should send:

XXX

Belliol college

Wide street (in fact, it doesn't matter whether this is available or not, the postman knows it)

Oxford

Ox 1 3bj (postcode of Baliol)

If you send it to someone in the math department, it is:

XXX

Institute of Mathematics

24-29 St. Giles' (in fact, whether this is available or not, the postman knows)

Oxford

OX 1 3LB (zip code of Mathematics Department)

But generally speaking, only teachers and some students who do TA have files there, so if you send a letter to a student, you should send it to his college. In addition, Oxford is not divided into colleges according to departments, so we should pay attention to it clearly.