You all moan on the CoC and OOB in Hoi III...as player of Gary Grigsby's War in the East and War in the West I can tell you, that Hearts of Iron III the OOB was okay. I like the way, how the Gary Grigsby''s games handLe it and it would be a pity in my opinion, if Hoi IV would not get an expansion that at least would expand the CoC to Corps. It doesn't have to be on map at all.
In WitE the OOB actually matters because it controls the flow of supplies/reinforcements and has support units attached at the corps/army level. HoI3 never supported corps-level support battalions and the supply system was such a mess that I honestly couldn't tell by what rules it worked. The result was an OOB that added very little to the game except immersion, and the amount of work necessary to keep it functional outstripped the benefit from the added immersion by far.