Q: Can i use retail version of A2 with my STEAM A2: OA ?
A: Yes, You can as long as the language is supported and ARMA 2 is updated to version 1.08
Note: might not work with Polish and Russian retail releases
This is form an old official post in the steam forums (so forget about 1.08 version). It shows you it works fine.
I guess you open and close first arma 2 so it copies the entry to the register. Then you open OA from steam and Arma 2 should be listed in the expansions menu. Activate it and after restart you should be using Arma 2: CO