#1 Make sure you run both arma2 and arma2:OA from steam once.
#2 Download
Worrom - Work in progress and point it to your steam directory. The default path is C:\Program Files (x86)\Steam\steamapps\common\arma 2 operation arrowhead\. If you have more than one hard drive, make sure you assign the right letter (F, G, etc).
#3 Download
BattlEye - The Anti-Cheat Engine and run the CLIENT (BE Client) NOT the BE Server.
#4 Open up the worrom tool you downloaded in step 2. Click "Check for update". Wait for it to finish, if success, hit the play button in the worrom tool. If failure, hit Force Update in worrom tool.
If you encounter a pixel shader error (you may or may not) the fix for that is to run Arma2:OA as ADMINISTRATOR. To do that right click the icon on your desktop and select "Run as administrator"
I hope this helps. If you are still having trouble i'll bring you in vent with me and we'll troubleshoot further.