O'Donnell said she did not post the extensive LinkedIn profile, which includes in-depth descriptions of several jobs she has held and her regular appearances on Bill Maher's "Politically Incorrect" show in the 1990s.
The tea party-backed Republican has been under scrutiny since she pulled out a surprise primary victory over moderate Rep. Mike Castle. She now faces Democrat Chris Coons in the November general election. Castle said late Wednesday that he would not wage a write-in campaign.
O'Donnell was at Oxford for three weeks in 2001, attending a summer course called "Post Modernism in the Millennium" sponsored by the nonprofit Phoenix Institute. Christine Fletcher, who oversaw the program, said it featured several lecturers who were members of the Oxford faculty, as well as U.S. academics.
She recalled O'Donnell as a good, attentive student who once organized a dinner for fellow participants.
But she also said it could be misleading for anyone who attended the program to say they had studied at Oxford University.
"It depends on exactly how you phrase it," she said. The education section of the LinkedIn profile reads simply "University of Oxford," followed by the name of the course O'Donnell took.
The LinkedIn profile has a similar listing stating that O'Donnell studied constitutional government at Claremont Graduate University in California, when actually she completed a fellowship at The Claremont Institute, a conservative think tank in the same city. Rod Leveque, a spokesman for the university, said it is not affiliated with the Claremont Institute and had no student or education records for anyone named Christine O'Donnell.
O'Donnell insisted Wednesday in a statement released by her campaign that she never established the LinkedIn profile or authorized anyone else to do so. She called reports that she had released false information "categorically untrue" and asked LinkedIn to remove the profile.
"I have always been clear about my educational background," she said.
Eric Glass, a spokesman for LinkedIn, said Wednesday that the profile had been removed.
"To protect user privacy, we can't share any further information about it with you," Glass said.
Chris Merola, a spokesman for O'Donnell, said Wednesday night that the campaign hadn't decided yet whether to investigate where the posting came from or whether to press LinkedIn to find out who was responsible.
Wednesday was the first time O'Donnell's campaign challenged the authenticity of the profile, even though a spokeswoman was asked about it last week by Greg Sargent, a blogger for The Washington Post.
Asked Wednesday to explain why she did not challenge it when talking with Sargent, O'Donnell spokeswoman Diana Banister replied simply, "Ms. O'Donnell has clarified any questions about her education and the LinkedIn page."
Sargent noted Wednesday on his blog that Banister earlier sought to explain the Oxford listing by pointing to O'Donnell's Phoenix Institute course.
O'Donnell has made incorrect or misleading statements about her education before. After claiming for years that she had earned a bachelor's degree in 1993 from Fairleigh Dickinson University in New Jersey, she received the degree only this month. The university had withheld it and sued her because of unpaid bills.
In a 2005 lawsuit that O'Donnell filed against the Intercollegiate Studies Institute, a conservative think tank where she worked less than a year before being fired, O'Donnell said her bosses reneged on a promise to allow her time to pursue a master's degree at Princeton University.
The lawsuit, which O'Donnell filed herself, claimed she had applied to Princeton for the fall 2003 semester to "work toward a master's degree," but that she had to withdraw from classes after enrolling and ask Princeton for a tuition refund.
After filing her lawsuit, O'Donnell hired an attorney who submitted a revised complaint, which also said she had applied for admission to a master's degree program at Princeton.
But asked by The Associated Press earlier this month about her educational record, O'Donnell denied she ever enrolled in a master's degree program at Princeton.
"I wasn't pursuing a master's degree. I was taking an undergraduate course at Princeton," she said.
Reminded about the assertions in her lawsuit, O'Donnell replied, "Well, then my attorney got that wrong."