The President's office was a room in the Capitol provided as quarters for the Lord President. Situated on level 5, it was connected to the Chancellor's office by a secret door. The door opened when the words "there's nothing more useless than a lock with a voice print", a popular saying of Borusa's, were spoken.
The Fourth Doctor had the room lined with lead during his time as President to prevent the Vardans from reading his thoughts or those of anyone else in the room. He used the secret door to access the room from the Chancellor's office, discovering the key while reciting Borusa's adage.
While in the office, K9 Mark I was given the Sash of Rassilon, the Rod of Rassilon and the Crown of Rassilon by the Doctor. Leela, with Rodan's assistance, led the Outsiders to the Doctor and they were present in the office when K9 sent the Vardan's back to their homeworld.
When the Sontarans invaded, Borusa made his way to the office through the secret door and used the celebration chimes, amplified fifty times in the Doctor's estimation, to stun the Sontarans long enough for the Doctor his party to escape. They made their way to the office where they met with Borusa.
The Sontarans attempted to force the door, but Borusa revealed he had had it lined with a titanium based alloy for protection. The group evaded capture by leaving through the secret door. The Doctor unlocked the usual entrance, allowing Stor to enter and discover the office empty. (TV: The Invasion of Time)