I was getting the exact same error message yesterday, along with another error message right after it "Error compiling pixel shader PSSpecularAlpha:0"
I don't know the exact reason why but to fix the issue I used steam to "delete local content" for both Arma 2 and OA. I did a search for anything Arma related and deleted it all. This includes the Arma folder in Program Files/Steam/steamaps/common and there's an Arma folder in the documents folder as well. Both deleted. I then made sure I restarted steam "as administrator" (this could have been my problem) and installed Arma 2, ran it, installed OA, and it ran without those two errors.
Now I have a completely different issue with both Arma 2 and OA constantly freezing on me for a couple seconds every few seconds. Makes the game unplayable...... buuut, that's another topic.