Prior to the west wing and Oval Office being built in the early 1900s, where was the Presidents office?

Answer 1

Actually, there were two oval offices. Theodore Roosevelt first used the West Wing for executive offices. Taft built an oval office in the middle. Franklin Roosevelt moved it to the current location. Before that rooms of the Residence part of the building were used. Staff was considerably smaller then. Lincoln had a very small staff and wrote most of his communications personally. The Halleck book will probably give you a picture of the White House inner workings during the Lincoln administration, Halleck was the chief of staff.

Answer 2

I think it was in the building that is referred to as the "OEOB" or "Old Executive Office Building" which is across the alley from the where the West Wing was built.