When I went for my third interview (at Oriel) the other person called for interview there from Brasenose wore a suit. I wore a hoody, jeans and a smartish top underneath. It's December for goodnesssake warmth over style at all times. I just took my hoody off before I went in the room and tied it around my waist. You don't have to be dressed up for the interview at all.
As for the open day I wore a vote Kermit T-shirt and jenes and obviously it didn't have any effect on me getting an offer.