Read, read, read, read, read, and then read some more. Also: read.
Studying history at Oxford is all about guided self-instruction. Tutors prefer to be the pilot not flying with the student as the handling pilot. The Tutor may only briefly touch the controls if the aircraft encounters turbulence or gets too slow on the approach to land. Socratic method is in abundance - learn by robust debate. Try to be comfortable with irony, and even sarcasm. Withering Oxbridge scorn is a thing.
Historians tend to live in the Upper Reading Room of the Radcliffe Camera, a good place to be, as is the Upper Reading Room in the Old Bodleian. The All Souls Library is also a good place to get some work done.
The default standard amongst your fellow students will be excellence and enthusiasm. Nobody is at Oxford just for the parties and punting (but don't miss out on the parties and punting).
To be an Early Modernist at Oxford, it's a good idea to be familiar with the work of Laurence Stone, Christopher Hill, Hugh Trevor-Roper, and Keith Thomas. To this list I add my tutor Cliff Davies. His mid 60s essay on The Pilgrimage of Grace is still great.
The attached article has lots of interesting stuff about history in Oxford.
https://academic.oup.com/past/article/261/1/259/7246025?login=falseSee if you can obtain an invitation to join The Stubbs Society - an inter-collegiate historical discussion group.
You can participate in the life of your college as much or as little as you choose. If you want to interact with the undergraduates, you could apply to be Sub-Dean (the person who is supposed to police the partying). At my college we had a Rhodes Scholar from Princeton to whom Juvenal's words
quis custodiet ipsos custodes?* could be applied.
Have fun!
PS: Did I mention reading?
*Who watches the watchmen?