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]

Moby Thesaurus II by Grady Ward, 1.0:

109 Moby Thesaurus words for "oddball":
   abnormal, absurd, alien, anomalous, anomaly, bizarre, black sheep,
   blemish, case, character, crackpot, crank, crankish, cranky,
   crosspatch, crotchety, curious, deviant, deviative, different,
   divergent, dotty, duck, eccentric, erratic, exceptional, fanatic,
   fey, flake, flaky, foreign body, foreign intruder, freak,
   freaked out, freakish, freaky, funny, hermit, hobo, idiocratic,
   idiosyncratic, impurity, individualist, inner-directed person,
   intruder, irregular, kinky, kook, kooky, lone wolf, loner, maggoty,
   maverick, meshuggenah, misfit, monkey wrench, mote, natural,
   naysayer, nonconformist, nut, nutty, odd, odd fellow, oddity, off,
   off the wall, original, out, outlandish, outsider, pariah,
   passing strange, peculiar, quaint, queer, queer duck, queer fish,
   queer specimen, quirky, quiz, rara avis, recluse, screwball,
   screwy, singular, sliver, solitary, speck, splinter, spook, sport,
   stone, strange, strange duck, tramp, twisted, type, unconventional,
   unearthly, unnatural, wacky, weed, weird, weirdo, whimsical,
   wondrous strange, zealot, zombie