Does this really mean that their office is on the ninth floor?
In American English, ‘the eighth floor’ means the floor seven floors above the ground floor, while ‘the ninth floor’ means the floor eight floors above the ground floor. Which of these does ‘o oitavo andar’ mean in Brazilian Portuguese?
In Portuguese, "o primeiro andar" is what an American from the United States would call the second floor. U.S. Americans call the ground floor the first floor, and Brazilians call it "o piso térreo." Therefore, yes, "no oitavo andar" would be on the ninth floor if you were translating for a U.S. American.