Chief Justice Roberts botched it. From the US Constitution, here is how the oath reads:
Before he enter on the execution of his office, he shall take the following oath or affirmation:--"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."
See all 1084 reactions on YouTube..
UPDATE 22 JAN: Obama retakes oath of office after Roberts' mistake - CNN