You'll probably never have more than 4 contact hours a week so you'll spend most of your time reading in libraries. In short, most of your learning will be done independently and alone. Some people thrive with this, others find it frustrating and difficult to motivate themselves. History has a reputation for being a bit of a doss subject, with historians being those who do least work. This is not entirely true, of course, though history is perhaps one of the less demanding subjects at Oxford. In other words, you can get away with doing little work if- nobody has failed first year exams since WWII. Of course, the majority still work fairly hard. With the lack of structured teaching, history at Oxford really depends on what you make it, and what you do with your time away from studying.
As for advice, do well on the HAT and in interview you'll get a place. Practice the HAT and ask for feedback (though how to write a good answer is anyone's guess). Make sure you know your A-level content well should they ask you about it in interview (especially the topic you wrote your submitted essays about). Try and think about history as a discipline itself as well - not just about specific time periods.