"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."
Article II, Section 1 of the United States Constitution
Barack Obama will take the oath behind bullet proof glass with his hand on a Bible used by Abraham Lincoln. John Roberts, Chief Justice of the Supreme Court, will administer the oath. Senator Obama voted against Roberts' confirmation. Although not part of the constitutional oath of office, beginning with George Washington, it has been a tradition to add "so help me God" at the end.
Technically, it isn't taking the oath of office that will make Barack Obama President of the United States - it is high noon. The 20th Amendment states "The terms of the President and Vice President shall end at noon on the 20th day of January ... and the terms of their successors shall then begin." Obama should take the oath about 11:58 AM, but even if things are running a little behind schedule, he will be President, and George W. Bush a private citizen, precisely at noon.
Absent from the Inauguration Ceremony and other events will be Secretary of Defense Robert Gates who is at an undisclosed location to serve as Obama's "designated successor" ensuring continuity of government if a catastrophe occurs. Gates is the only member of the Bush Cabinet who will also serve in Obama's.