Here’s the text in Article II of the Constitution:
"I do solemnly swear (or affirm) that I will faithfully execute the office of President of the United States, and will to the best of my ability, preserve, protect and defend the Constitution of the United States."
Here’s Obama’s oath, as spoken:
"I, Barack Hussein Obama, do solemnly swear that I will execute the office of President of the United States faithfully, and will to the best of my ability, preserve, protect and defend the Constitution of the United States, so help me God.”
Obama's spoken misstep was the misplacement of the word “faithfully.” (Roberts also said “president to the United States,” but Obama wisely did not repeat the incorrect “to.” )
A cursory look at past inaugurals reveals that Obama is not the only one to take a slightly altered Oath.
Dwight Eisenhower added an extra “the,” saying: “...the office of the President of the United States...” in 1957.
In 1965, Lyndon Johnson said "the Presidency" rather than just "President," saying "...the office of the Presidency of the United States..."
And in 1973, Richard Nixon added an "and," saying “...preserve and protect and defend...”
ABC notes that in 1909 William Howard Taft also misspoke, saying "preserve, maintain and defend the Constitution."
Clearly, these little errors didn't stand in Taft, Ike, LBJ, or Nixon's way of serving as a legitimate president (well, OK, but Nixon's legitimacy problems had nothing to do with his Oath!). Taft even went on to become Chief Justice himself.
Two other tidbits: George H. W. Bush also started to interrupt during the first clause, just like Obama; and try repeating the whole thing at once, like FDR had to do in 1933!
One last comment about the Oath today: Did anyone else find it slightly offensive the way Roberts asked Obama "...so help you God?" as if to question Obama's faith? Of the Oaths I listened to, Roberts is the first to pose it as a question like that.