Ms Napuk, who came from Edinburgh, was in her third year studying history at Lady Margaret Hall and was due to sit her final examinations in six weeks' time.
Sir Brian Fall, her college principal, said they were "devastated". She was an "outstanding student" who had been immensely popular and always helpful.
A university spokeswoman said Ms Napuk had been "experiencing difficulties" and had been in touch with the university's medical and counselling services
It is the second time in recent years that the college has been hit by tragedy. In October 1992, Tracey Cole, 18, was found hanged in her rooms a week after arriving from her home in Exeter.
The principal of The Mary Erskine School, in Edinburgh, where Sarah Napuk was head girl, said everyone there would be "devastated" by her death.
Patrick Tobin described her as a "wonderful girl". He added that she had "an extremely sharp intellect ... she made things happen".
Mr Tobin said that after she left school Sarah had spent a year in China working for a charitable organisation and at Oxford had continued and developed her concern for those less fortunate than herself.
Sarah's brother David, 20, told the Edinburgh Evening News he thought exam pressure had contributed to his sister's death. "She was not so much a brilliant student as a brilliant person. She meant so much to all the people she knew," he said.
"She gave so much love to everyone. I think she had her own reasons for doing it but I do not want the world to read about that."
Sarah was engaged to fellow student Jason Russell and planned to go to Harvard once she finished her degree.Reuse content