This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
if "o" is of type Object then, how (o instanceOf Moof ) is true,
No, "o" in here is a method local variable, think of it as a place holder. At the runtime the code calling this method passes a reference which might/might not be of type "Moof". Hence it should be verified before proceed further. Read more about variables here.