What you said
The withdrawal room is good as a safe base, but it's not a place where he is able to learn in a mainstream class. I would put him with small groups and build up slowly.
The expert view
The pupil should understand that joining the group and access to play is in some way conditional on better behaviour - a diagnosis of ODD is just a description of the behaviour rather than an identification of a medical problem. It's risky to treat this as something the child can't help, because that wat the child might learn that others will accept that this conduct is inevitable, or in some way acceptable.
This is part of a larger debate - the current obsession with medicalising behaviours because they seem extreme-spectrum. In any population there will be minorities at the fringes of the norm. If minority traits produce desirable effects (intelligence, say), we applaud them; but if they produce effects such as ODD, we tend to reify them as illnesses.
Children who would once have been encouraged out of their behaviours are now appeased and stripped of responsibility for their actions. Treat this boy as if he were the master of himself. Encourage him through patient reward and reinforcement to take part in mainstream society. Otherwise he risks falling into criminality and failure.
This could be a vital intervention. He isn't sick; he is badly behaved. You can't control his mind, but you can help him learn to control himself. Praise the good, sanction against the bad, and help him re-enter the classroom. Try small-group reintegration, but create a behaviour contract and ensure it has clear outcomes. He needs tough love now, almost as much as he needs oxygen.
Tom Bennett is author of `The Behaviour Guru' and `Not Quite a Teacher': http:behaviourguru.blogspot.com
Post your questions on: www.tes.co.ukbehaviour.