I know I am repeating myself, but about the closest you can get to Harry Potter in Oxford is taking part in a “Formal Dinner”. These are held in the colleges, mostly on a weekly or more than weekly basis. In general, this is a dinner that – unlike other dinners – takes place in a more formal (who would have thought it?) atmosphere. Lights are dimmed, candles are lit and the three-four courses are brought to your place. Behave well though (for instance, our students were told off for taking selfies; rightly so as far as I am concerned)!
(newliturgicalmovement.org)
Usually, there are seats and dining tables for ordinary students and their guests on the one hand and for professors and college officials (the “High Table”) on the other. Only after the former have taken seats will the latter enter the dining hall. Students will then get up and wait for the High Table to be seated and a prayer and the college grace to be spoken. 
We were the first to enter hall on this occasion at University College
Dress codes differ between colleges. I think it is safe to say that in most cases an ordinary suit (not necessarily a tie) will be sufficient. Oxford students are expected to wear their gown. Some special dinners (or in my college’s case “Formal Formals”), however, require “Black Tie”. While this may feel a bit strange at first, it is definitely great fun to dine with all of your college dressed up.
(aiesecinoxford.wordpress.com)