My 8 yr old was diagnosed with bi-polar and adhd this year. One of the things that has been brought up in the research I've been doing is that there is a disagreement brewing in the psychiatric community over whether ODD is an actual disorder in itself, or a symptom of something else. One person said it would be like saying a blind person has "Walking into Walls Syndrome" - it just adresses a single symptom, and not the cause of the symptoms. Their point was that a diagnosis of ODD is just not enough - the cause of the ODD needs to be treated.
You don't mention if he is on any meds. My son is on Risperdal and it was absolutely amazing the difference in him just the first week he was on it. The medication made it possible for us to discipline him. Before that, all we were really doing was restraining him until the episode passed. Now if we tell him he needs to go to his room, he actually goes. We used to have to pick him up and carry him there, and then hold him down to keep him there. At age 2, he grabbed a knife from the dishwasher and chased his older brother through the house. At age 4, he tried to hit me in the head with a putter while playing mini-golf. Last fall, he tried to hit me in the back of a head with a decorative marble egg. We have holes in walls and broken furniture due to his outbursts. We were even kicked out of a Bible study group after he kicked one of the teachers there.
I encourage you to find a psychiatrist who works primarily with children, and continue to press on until you can get your son some help.
I wouldn't worry about him repeating kindergarten. Most boys do better waiting to start kindergarten, particularly if he has a late spring or summer birthday. My son has several developmental delay "gaps" - because he spent so much time trying to deal with what was going on in his own brain, he couldn't work on the developmental step that others his age was working on. Now that he is stable and getting the right kind of help at school and home, he is on his way to overcoming those. I expect him to catch up with his peers within a year if he stays stable. If I had waited much longer - or accepted the word of the first psychologist we took him to who said he needed 7 minute timeouts since he was 7 years old @@ - he would be further behind and have more to overcome.
Keep pressing on. Check with your local mental health department to see if they have any programs for children with emotional disorders, or support groups for the parents. You can connect with other parents and get the names of responsive psychiatrists and possibly even find some respite care so you can focus on your girls or yourself for a few hours. Your daughters can find other kids with "different" siblings who understand what they are going through, too.
Blessings to you.