It's been awhile since I've had to start over from scratch on this, but I believe the best way to do it would be to use the Steam client to download both games and the dedicated servers for both games (from the tools menu). Then, move or hardlink the arma2 files into a "arma2" folder on the ArmA2:OA installation. Finally, use a command line that includes this, in addition to the standard options:
For instance, the final line might look like:
ARMA2OASERVER.exe -mod=arma2;expansion;ca -port=2302 -name=server -config=cfg\server.cfg -cfg=cfg\basic.cfg -profiles=cfg
This would need to be run from "C:\Program Files (x86)\Steam\steamapps\common\arma 2 operation arrowhead", most likely. You would need to put a valid server.cfg and basic.cfg in your "cfg" folder before doing this (I'm not sure if they're included).
More information on command line options can be found here: http://community.bistudio.com/wiki/Arma ... Parameters