Another problem is how the lower portion of the ship can be accessed. Of course, this also depends on what is actually located down there. There are no windows and nothing else that could indicate that it's a permanently inhabited area. The deuterium tank may occupy the upper part. If the engine room is in the lower hull, the power transfer conduits would have to go up all the way from the lower hull to the nacelles. On the other hand, if the warp core is located in the upper hull, probably both the matter (from the presumed deuterium tank) and the antimatter would have to be routed through the pylons. In either case the Oberth is a poor design.
No matter if the lower hull has to be frequently accessed or only a few times per day, we wouldn't consider that the transporter is used for that purpose each time. Especially if we consider that the Oberth is a dated design, we have to keep in mind that, at the times of TOS, intra-ship transport was regarded as dangerous (mentioned e.g. in TOS: "Day of the Dove"). There may be some sort of dedicated transporter channel between the upper and the lower hull to minimize the risk, but there's no reason not to make the pylon wide enough for a small vertical turbolift channel instead. Yet, if we postulate that the turbolifts are running through the pylons, we get serious problems. The pylon thickness appears to be less than 1.8m on the Fact Files front view, and the actual photos support this impression. A tiny 1m x 2m turbolift car could barely fit through such a channel, and it would be impossible to make it follow the curves, let alone technically sensible. Moreover, the car would depart vertically in the upper hull and arrive horizontally in the lower hull, and it would have to be turned by 90° again upon arrival. The change of the direction of gravity, on the other hand, would be no problem, considering that the car has its own gravity generator. Anyway, the designer of such a turbolift must be a complete idiot! Or does the ship rather have a pneumatic delivery system like in "Futurama"? ;-)