I was trying to run OA to play Wasteland Ap0c, it got as far as the Bohemia Interactive logo then dropped to desktop.
I'll give it another go and just make sure I backup all the files I change this time.
Added 32 minutes later:
So I deleted ArmA 2 & Operation Arrowhead out of Steam and re-downloaded the pair of them yet it still is giving me the same error as if I'd not even un-installed.
The error is ;