Steam

You do not have to be always online. You can launch the game with Steam in offline mode.

If you want to switch to the offline mode please click on Steam (top right corner in your Steam window).

Select "Go Offline..."

And restart your Steam client.

Please note that you will not be able to play multiplayer while in offline mode.

You can also switch auto-updating feature of the Steam off:

Right click the game in your Steam library.

Select "Properties"

Under the "UPDATES" you can change settings for automatic updates

Please note that most of the servers will run on the latest version of the game.

Run Steam as Administrator (right-click on the Steam icon and click on "Run As Administrator").

Install Arma 2.

Run Arma 2 once.

Install Operation Arrowhead.

Run Operation Arrowhead through Steam (Battleye will now install automatically).

If you wish, you can also install the Arma 2: Operation Arrowhead beta (for the purposes of running Mods etc.). It will appear as a separate item in your Library.

There are several possible fixes for this issue.

First, run Arma 2 once. Then try to run Arma CO (without beta or DayZ).  If that won't help you can try to verify game cache of the Arma 2 and Arma OA.

1.) If that will not help, follow this guide:

1. RUN STEAM AS ADMINISTRATOR
2. Exit Steam.
3. Right-click on Steam icon.
4. Left-click on "Run As Administrator".
5. If the problem persists, reboot your computer and try the procedure again.

2.) DELETE ARMA 2 CO REGISTRY ENTRIES

1. Exit Steam, open Start menu and type this directly into the menu (in Windows XP, you will have to click on "Run" first) regedit
2. Press Enter.
3. In regedit window, click on HKEY_LOCAL_MACHINE.
4. Press CTRL+F to bring up search.
5. Look for "ArmA" (without the quotes).
6. Delete all the "ArmA" entries you find from the registry.
7. Run Steam as Administrator.
8. Right-click on the game in Steam Library.
9. Click on "Properties".
10. Select "Local files" tab and click on "Verify integrity of game cache files" button.
11. Please wait, the process can take several minutes.

3) REINSTALL STEAM

1. Go to Steam's installation folder.
2. Delete everything there except "steam.exe" and "steamapps" folder.
3. Run "steam.exe" - Steam will reinstall itself automatically.