JAMF, is your OA installed "inside" your ArmA2 root directory?
If not, which root directory has your addon maps? OA or ArmA2 root?
Separate, as Steam automatically does:
D:\GAMES\Steam\steamapps\common\arma 2
D:\GAMES\Steam\steamapps\common\arma 2 operation arrowhead
Since the addon maps are working in ArmA2, I am suspecting they are in ArmA2's root directory. Try moving them to the OA root directory, if this is the case.
I did just that:
I finally copied the islands into the Operation Arrowhead map, so they show up in the editor.
How are you starting combined ops? Through the Steam shortcut? Or are you using the CMD file I made? If so, did you edit that to point towards your actual install folders?
I did start combined ops, by starting OA, then selecting the CO option.
When I tried using the CMD files, with the correct paths, but they resulted in the MPTable.Shadows and pixel-shader errors.
I can send you also my modified "combined ops" STEAM start file. I got it from some great people on the BIS forum. It is setup for Win7x64.
But first you need to delete all the launch options. Don't use that. It is set in the .cmd file.
Cool, I'd like to try that.

If you could send it to my jamf72 at hotmail dot com address, that would be great.
[EDIT] Kicked off Tao's version of Evo on the SimHQ server. I think I'll be running around in it till around
3AM local / 2AM UK time 23:00 Zulu/UK time.