If you only have operation Arrowhead, you can download Arma 2 free and link it to Operation Arrowhead to get Combined Operations lite.It's really quite simple.Go to steam and right click on Operation Arrowhead, got to 'properties'In the 'advanced' tab, click 'set launch options'a small box with a single line to write in should appear, type this into the box.
arma2oa.exe "-mod=c:\location\of\ArmA 2 Free;Expansion"
obviously change "location\of\arma 2 free" to the actually location of it. If it's on steam then it will be in programfiles(x86)\steam\steamapps\common\arma2free
or something like that.
Then launch operation arrowhead and you will be running combined operations.
The only difference is that you do not have the Arma 2 campaign and the Arma 2 models are of a slightly lower quality.