The Collaborative International Dictionary of English v.0.48:

oddball \odd"ball`\ ([o^]d"b[add]l`), n.
   1. A person with an unusual or odd personality; an eccentric
      person. [informal]

   Syn: eccentric, eccentric person, geek.
        [WordNet 1.5]

   2. Hence: Anything unusual in its class.
      [PJC]

            Pluto is an oddball among its eight sister planets.
            It's the smallest in both size and mass, and has the
            most elliptical orbit. It moves in a plane tilted
            markedly away from the other planets' orbits.
            Moreover, Pluto is the only planet made almost
            entirely of ice.                      --Ron Cohen
                                                  (Science News,
                                                  Feb. 27, 1999,
                                                  p. 139)

The Collaborative International Dictionary of English v.0.48:

oddball \odd"ball`\ ([o^]d"b[add]l`), a.
   Eccentric; very unusual; strange; bizarre; as, an oddball
   request. [informal]
   [PJC]

WordNet (r) 3.0 (2006):

oddball
    n 1: a person with an unusual or odd personality [syn:
         eccentric, eccentric person, flake, oddball,
         geek]