I wouldn't have started a new thread if there wasn't at least *one* important difference here to be aware of!
This applies to me and probably anyone else trying to map with ArmA 2: Combined Operations purchased off of *steam*. It might be the same if you bought it elsewhere, YMMV.
While I was setting up my work directory, specifically my P:/CA drive, I unpbo'd and moved all the important bits from both my Operation Arrowhead "Common" folder and of course the "Expansion" folder.
"Common" is where it gets tricky! At first glance you see what you would think would simply be copies of essential pbos common to both OA and A2. They even have the same names, such as "misc3.pbo". However these are NOT the same pbos! They aren't even close to being the same size, usually the A2 versions being MUCH larger!
Mikero pointed this out to me and suggested I try first unpbo'ing the ones found in my oa/common folder and then overwriting them with the ones from arma2/addons, so this is what I have done.
Testing is still going down on this so if anyone has any feedback or suggestions feel free to pitch in!