Head Master's Office
"She proceeded to a gargoyle in the halls, and says the password ("Sherbert Lemon"), whereupon the gargoyle lept aside, revealing a slowly ascending circular staircase. Reaching the top of this, they found an oaken double door. Professor McGonagall left Harry there. Harry saw that the room contains a vast number of portraits of past headmasters, all evidently asleep; many spindly tables with intricate silvery devices upon them; and the Sorting Hat resting on a shelf. After exchanging some words with the Sorting Hat, Harry notices a very ill-looking bird on a perch behind the door; as he watches; aghast, the bird catches fire. Professor Dumbledore, then entering, tells Harry that the bird, Fawkes, is a Phoenix, and that is his method of immortality; he will shortly be reborn from his own ashes."
This is the Office of the Hogwarts Headmistress, Kat Von D. This is always the office of the current headmaster or mistress of Hogwarts. It is located in the Headmaster's Tower