Actually, if you install ArmA II and ArmA II OA Afterwards, it should create a link for CO automatically, so basically you launch ArmA II OA, but with all A2 features such as USMC, NAP, Chedaki ect. as playable units.
If that doesnt work, i've found these videos, they might help?
Youtube: How to Combine ArmA II and Operation Arrowhead.
Youtube: Arma 2 Combined Operations Steam WORKING FIX
Hope it helps, good luck! (If everything fails, you might as well request help from the Steam support, because usually those games combine automatically, i guess if they don't, thats a Steam issue.