& = And the following conditions together
objectCategory = is it a user, computer, etc.
operatingSystem = what is found in the computer object operating system tab "name" field
! = Condition is "Not"
memberof = Find members of the group
1.2.840.1135126.96.36.1991 = Tells the lookup to recurse the member groups of the super group
You must use the full distinguished name of the group in question.
Of course you can adjust this to specific OS versions or group names or even extend it to include additional references to more groups.