You're likely going to need a full team on this - ODD is kind of a catch-all, and most of us believe the docs stick it on there when they don't really have answers, but it doesn't get you any help. Anything could be going on, from bi-polar to Reactive Attachment Disorder (RAD) (considering all the back-and-forth and mum not paying much attention in his youth, etc). Until he gets a full evaluation from a neuropsychologist and/or multidisciplinary team, you won't really know. Don't forget to check into things like wheat allergies, etc., too. Those can also show up as behavior problems (or increase existing ones).
Living with him also places additional stress on you and your daughter, so also check into family therapy as well as therapy for your son. Does he have an IEP for school? If not, start the ball rolling to get him one by requesting it in writing, through the mail, return receipt requested. This puts them on a federal deadline to evaluate him for services, and while that's happening make sure you get an experienced advocated for the IEP meeting (many schools will try to get by with the least amount of help possible if you don't have one).
Don't be afraid to call your local child protective services and ask them for help and resources - they have a lot available, often including in-home help that you don't know about if you don't ask.
