Any presidential historian knows that the Oval Office didn't exist until the Teddy Roosevelt administration, and, after that, changed locations in the West Wing (the original oval office was located in the center of the West Wing, but was moved by FDR to its current location at the southeast corner).
Bill O'Reilly, however, isn't a presidential historian.
“Killing Lincoln” makes multiple references to the Oval Office; in fact, Emerson points out, the office was not built until 1909. ... The book says that Ford’s Theatre “burned to the ground in 1863.” In fact, the fire was in 1862, according to the review.
Lincoln might have owned a time traveling DeLorean or something and traveled to the TR administration where he borrowed the West Wing for a while -- in fact, I wouldn't be surprised if this turned up in O'Reilly's book.
There was the Yellow Oval Room while Lincoln was president, but that was used by Lincoln as a library and was never an "office" during that administration.
I'm sure you've heard by now that Ford's Theater will not be carrying O'Reilly's fiction novel in its book store.