It's worth saying that while you'd be free to leave Oxford to go to an evening concert during freshers week, there's likely to be compulsory college-run events on until at least 5pm on some days, and missing these would likely get you off on the wrong foot with your tutors. If you are thinking about booking tickets, it's worth contacting your college now, to see if they can give you an idea of whether you'd be missing any compulsory events or not.