On my last free afternoon, I was walking by the Sheldonian and saw that it was open. The other times I'd been by there, it was closed. The building is basically a large auditorium.
When I paid my admission they gave me a booklet describing the degree granting ceremony that takes place here. Typical of Oxford, I don't think it's changed in centuries. There is a lot of processing back and forth, and bowing, and coming in one door and out another, and speeches according to a strict script--all in Latin! I can imagine the students thinking: "Is it over yet? Do I have my degree yet, or what?"
Christopher Wren had a problem when designing the Sheldonian. He modeled it on an ancient Roman theater, but that theater had an open roof--in the Oxford climate that was not practical. Wren had to come up with a way to put on a roof that would span the wide extent of the building, and support it without putting big columns into the auditorium that would obstruct the sight lines of the audience. He devised a system of huge wooden trusses in the roof, concealed from view by this fancy painted ceiling.