The Collaborative International Dictionary of English v.0.48:

Oaf \Oaf\ ([=o]f), n. [See Auf.]
   1. Originally, an elf's child; a changeling left by fairies
      or goblins; hence, a deformed or foolish child; a
      simpleton; an idiot.
      [1913 Webster]

   2. A clumsy or awkward person.

   Syn: klutz, clod, lummox, stumblebum.
        [PJC]

The Collaborative International Dictionary of English v.0.48:

Auf \Auf\ ([add]f), n. [OE. auph, aulf, fr. Icel. [=a]lfr elf.
   See Elf.] [Also spelt oaf, ouphe.]
   A changeling or elf child, -- that is, one left by fairies; a
   deformed or foolish child; a simpleton; an oaf. [Obs.]
   --Drayton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

oaf
    n 1: an awkward stupid person [syn: lout, clod,
         stumblebum, goon, oaf, lubber, lummox, lump,
         gawk]

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "oaf":
   babe, baboon, beast, blockhead, blunderbuss, blunderer,
   blunderhead, boob, booby, boor, botcher, bruiser, brute, bull,
   bumbler, bumpkin, bungler, child, child of nature, chump, clod,
   clodhopper, clodknocker, clot, clown, dolt, dolthead, dove, dub,
   dupe, fathead, fumbler, galoot, gaum, gawk, gawky, gorilla, gowk,
   hayseed, hick, hulk, infant, ingenue, innocent, klutz, lamb,
   lobster, looby, lout, lubber, lug, lummox, lump, meathead,
   mere child, missing link, noble savage, ox, palooka, rube,
   simple soul, slob, slouch, slubberer, unsophisticate, yokel