Alright guys, your key was changed by a key stealer with a Invalid key or...something just went wrong :P, First go to Steam and launch Arma 2 OA. After you get in, Leave. Then launch Arma 2 and when you get in, leave. Now if that doesn't work go to RegEdit and find Bohemia Interactive Studio (For 64 Bit) and click ArmA 2 OA and then right click KEY which should be in the right box... and click MODIFY...and enter your CD-Key, Now if you key Isn't Battleye Banned you can enter your key from Steam (Must convert to Hex first) or you will have to buy Arma 2 OA again , To buy Arma 2 OA again you can go to the DayZ Sell/Trade shop on these forums, OR do what I would recommend..go make a new steam account and buy Arma 2 OA for 20$ (Not Arma 2 CO again). Trust me if you buy from the keysellers on here it will be banned almost exactly after there "Warranty" is up because they want you to waste more money being keys that will be banned in a day. Trust me you will spend more money on CD-keys from these sellers than just buying it once off of Steam. Now, If you plan on Hacking go for it, go waste a bunch of money on Keys from Key Sellers :P

If you have any other questions or problems I can make a quick video on how to do it if you want, or answer any questions you may have.

Thanks, Storm