Oddboy is the term used by the Orks to refer to any of their breed who, due to some difference in their genetic makeup, doesn't fit the standard template of the Ork Boy. The following breeds are currently established as Oddboyz:
Slaverz, also known as Runtherdaz, have a deep fascination with "runts" (most literally gretchin, but also any slaves the orks may have captured), and so they happily spend the majority of their time working with them. Slaverz keep the basic economy of ork society going, as they instinctively understand how to work a slave hard, but without killing it, and even have an instinctive interest in training them to work better.
Mekboyz have an instinctive understanding of, and fascination with, machinery and mechanical devices. They make up the ork's primary non-slave industrial class, churning out all the guns, tanks, rockets, etcetera that Ork society requires.
Painboyz are the medical class, having an instinctive understanding of basic medicine and a bone-deep obsession with exploring anatomy. The orks themselves are quite afraid of painboyz, for reasons that're obvious in hindsight.
Weirdboyz (also Wyrdboyz) are ork psykers, who have the unique ability to absorb the collective excitement and aggression of other orks and then channel it forth. Since they aren't exactly much smarter than regular orkz, they're not very adept at either understanding how they do this or how to do it safely. As a result, for the typical wyrdboy, it's a distressing, painful experience that can potentially be fatal, and so, unlike any other orkz, they prefer to avoid fighting wherever possible. The more masochistic Warp'eadz, on the other hand, have grown addicted to the feeling, and so happily seek out fights.
Speed Freekz are orks who have developed a fascination with speed over fighting. They still like to fight, but their obsession is with racing and going fast first and foremost. They make up the best bikers and aerial force members of ork society.
Sumboyz are not talked about much anymore; they're the ork word for "bankers" and "accountants". The joke in the name is that they're able to do actual math, and do counting more sophisticated than "wun... two... 'free... four... uh, lotz."
Some also argue that "specialist combat" type orks count as oddboyz too, such as Flash Gitz (who like big shootas over krumpin' stuff in melee) or Kommandoz (stealthy murderin' gitz) or Burna Boyz (pyromaniacs). Also of note are the Stormboyz (who have rudimentary uniforms they are expected to keep somewhat tidy, and run training drills).