[JURIST] US President Barack Obama [official profile] retook the presidential oath of office [text and materials] Wednesday, administered by Chief Justice John Roberts [official profile, PDF], following minor mistakes the two made in the wording of the oath during Tuesday's inauguration ceremony [text and video; JURIST report]. White House lawyer Greg Craig addressed the issue on Wednesday, maintaining that Obama was already properly sworn in [NYT report] despite the errors and that the retaking of the oath was only being done "out of the abundance of caution."
The oath of office is set down in Article II, section one, clause eight of the US Constitution [text] and has been taken by every US president [materials] beginning with George Washington. Chester A. Arthur and Calvin Coolidge [official profiles] are the only other Presidents to retake their oaths, although they both did so only because their initial oaths were sworn in private. Under the Twentieth Amendment [text], ratified in January 1933, the presidential transition actually takes place at Noon on the 20th day of January, regardless of whether the oath is sworn correctly or at all.