Some info how to combine these games can be found here:
http://forums.bistudio.com/showthread.php?t=101214
Or, if you prefer you can choose to reinstall.
First you install Arma 2!! Than you install OA inside the Arma 2 folder, by default OA will detect the Arma 2 folder automatically so if you just don't change the destination folder during the installation of Arma 2: Operation Arrowhead all should go by itself