Operation Arrowhead is the expansion
I've got Arma II and Arma II: Operation Arrowhead in my Steam games list right now. You had me scared for a second.
http://store.steampowered.com/sub/4638/
Sorry about that. Here's what happened.
The Steam page for the deal shows two products:
Arma II: Operation Arrowhead, and Arma II: Combined Operations.
No mention of 'Arma II' in the description or big print, only Arma II: Operation Arrowhead.
The quotes are all for 'Arma II: Operation Arrowhead'.
Then it says 'Arma II: Operation Arrowhead is a standalone product and does not require the original Arma II'.
I even read a review or II of it.
There's even DLC for Operation Arrowhead, 'British Armed Forces'.
Anyway, I had the wrong impression.
Looking at it now, 'Combined Operations' isn't a product, it's a package.
Sorry for the scare