C:\Program Files (x86)\ArmA 2\Expansion\beta
and drag arma2oa.exe to the desktop while holding ALT key this will create a shortcut on the desktop. (delete any other shortcut that may be created during installation, if any).
Now right click that shortcut and click properties, we are gonna edit the field "target" so it looks like this (with quotation marks and all):
"C:\Program Files (x86)\ArmA 2\Expansion\beta\arma2oa.exe" -beta=Expansion\beta;Expansion\beta\Expansion -mod=@DayZ -nosplash
Now we are going to edit the field "Start in" like this:
"C:\Program Files (x86)\ArmA 2\"