This is an image of the Divinity Schools (part of Bodleian Libraries). In the Harry Potter films, they look slightly different as there they are used as the Hospital Wing and are full of beds!
Original Image: Copyright © Oxford University Images / Photovibe -- All rights reserved.
This is Christ Church's dining hall, but it doubles up as The Great Hall in Hogwarts. Hope the real Oxford University students didn't miss too many lunches whilst the crew were filming!
Original Image: Copyright © Oxford University Images / Toby Ord -- All rights reserved.