Oval Office

The Oval Office in the White House was the main office of the President of the United States, and, later, the President of Earth.

In 1969, the Eleventh Doctor landed his TARDIS in the Oval Office, where he met President Richard Nixon. On confrontation, the Doctor claimed he was looking for the oblong room. The White House was put on lockdown because of the intruder. The Doctor slipped comfortably into Nixon's chair, claimed to be from Scotland Yard, and with the help of maps, helped explain the mysterious phone calls by a little girl the President had been getting. (TV: The Impossible Astronaut)

The President of Earth was based in the White House as of the late 26th century, (AUDIO: Scorpius, Fear, Conversion) and by at least 4041, the current President was based in the Oval Office. (COMIC: Return of the Klytode)

Behind the scenes
In Doctor Who in America, it was shown how the Oval Office was reconstructed on set, and Matt Smith revealed his wish for President Obama to allow shooting to actually take place at 1600 Pennsylvania Avenue.