Derived from the word Doppelganger, an Oppelganger is your doppelganger of the opposite sex.
Guy: If I was a chick, Reese Witherspoon would be my doppelganger.
Girl: Reese Witherspoon is totes your oppelganger.
someone who is seemingly just like you but on closer look is, in fact, your opposite.
My two blonde friends are oppelgangers--they're both blonde and have the same job, but one is a badass dry-witted sports nut, and the other is a fashion- and boy-obsessed neurotic.