for those who are getting a "wrong CD key" message do the following exactly!
1. Create a new notepad file.
2. Paste the following message into it:
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Bohemia Interactive Studio\ArmA 2 OA]
"KEY"=hex:86,0f,39,40,26,aa,1d,e6,d4,ea,8d,2f,5c,e8,c8
3. Name it regkey.reg
4. Patch the game