Longer focal lengths don't offer less DOF. If subject scale in the frame is the same for both a short and a long focal length, the DOF is the same for both lenses. That happens because to maintain the subject scale in the frame with the longer focal length lens, the camera has to be farther away from the subject.
However, because of perspective compression, the background the longer focal length renders, shows the same blurred background elements seen in the shorter focal length lens, as being bigger than the shoter focal length lens renders.