Yes, CO works fine if A2 is loaded as a mod of OA, but you get A2 showing twice in the expansion list. See shot below, which uses the launcher to launch CO, and I've selected ACR as a mod:

Code:
"-mod=C:\Program Files\Bohemia Interactive\ArmA 2;Expansion;ca;Expansion\beta;Expansion\beta\Expansion;acr" -nosplash -winxp -world=empty -maxMem=2047
With the above, everything will work fine, I believe, but it's annoying that A2 shows up twice (not a big deal though, obviously).
The issue is that the default CO shortcut, straight from BIS, uses this:
Code:
"C:\Program Files\Bohemia Interactive\ArmA 2\arma2OA.exe"
So, loading A2 as a mod of OA is not necessary to get CO to work perfectly; just run OA alone, and all is well (assuming that CO is installed properly).
Below is a shot of the expansions menu after using the launcher to start OA (NOT CO) with this modline:
Code:
"-mod=Expansion\beta;Expansion\beta\Expansion;acr" -nosplash -winxp -world=empty -maxMem=2047

Note that CO is loaded perfectly, even though A2 was not loaded as mod to OA.
The solution for the launcher is simply have two game versions available to start, A2 and OA, omitting CO. That is the functionality of alpinestars' launcher (http://arma2.paro-technology.com/).