You need Arma 2 cause the person who made the addon may have used material from both Arma 2 and Operation Arrowhead.
Arma 2 is really cheap these days. If you can afford a PC, an internet connection and OA and have the time to play games (meaning you don't have to work to earn money) you have also enough money to buy Arma 2 for like 15 euro or so.
Or, if you do not mind low res units you can combine Arma 2: Free with your Arma 2: Operation Arrowhead. And, Arma 2: Free is what it says.....for free.
Download Arma 2: Free from here:
- Arma 2: Free
And read this thread how to combine both games:
- OA + Arma 2 Free - For Combined Ops Lite