Supreme Court Chief Justice John Roberts mixed up some of the words.
The new president -- who taught constitutional law -- hesitated, apparently realizing the chief justice had flubbed.
On Wednesday, Vice President Biden joked about Robert's mistake.
"My memory is not as good as Justice Roberts," said Biden.
Biden then swore in about 100 senior White House staffers, many of them Chicagoans.
At that ceremony, President Obama signed executive orders toughening ethics regulations. New transparency rules apply to his government, him, and future presidents. He ordered a salary freeze for staffers making over $100,000 a year. And he tried to shut what's been called the revolving door between lobbying and government.
"Public service is a privilege. It's not about advantaging yourself. It's not about advancing your friends or your corporate clients," said President Obama.
Congressman Peter Roskam, a suburban Republican, was impressed.
"Clearly a good start. The next hurdle for him is this economic challenge," said Roskam.
That challenge was the topic of a morning meeting President Obama held with key advisers.
Another White House photo of the president's first day in the oval office shows him on the phone with middle east leaders, to promote Israel-Palestinian peace.
He also met with military advisers to discuss withdrawing US combat troops from iraq by mid 2010.
Later, he held an open house for residents of the District of Columbia. He greeted them, and offered a suggestion.
"Enjoy yourself. Roam around. Don't break anything," said Obama.
Some argue that Mr. Obama didn't need a do-over on the oath but two other presidents also had flubbed oaths: Chester Arthur and Calvin Coolidge. Both of them did repeat the oath later.
Also on Wednesday, a senior White House official disclosed that the president will sign an order on Thursday to close the Guantanamo Bay prison within a year.