Isn't the Federal Building still home to Federal courts?
Yes it is. It is the home to the Northern District of Oklahoma (non-bankruptcy) which has 7 judges and a support staff of ~35 people in addition to deputies in charge of security and as marshals in the Courtroom. If I remember correctly there are 3 main courtrooms on 4 and 2 courtrooms on 3 (never really stop to count them, just go to my room). Then you have judges offices (which generally have a front office, a clerk's office, and the judges office), a jury room for each court room, a conference room, a jury room, a small library, and the Clerk's office.
Also in that building is the Department of Justice. Never been in their offices. But it takes up about half of the third floor.
By and large, the building is as drab on the inside as it is on the outside. With the exception of some recently remodeled offices and the ceremonial courtroom.