Used to describe:
a) a idiot who feels they are significantly smarter than they are
b) a macfag
c) all the above
Can be used as a generic insult as well.
See that kid with his brand new macbook? Such an OMalley.
Person 1: Hey! I just got a 98% on that last test! Suckers!
Person 2: You're such an OMalley