My son has ADHD and Bipolar disorder and I know how you feel.
Here are some suggestions. Don't yell. Use touch and make sure he is actually listening to you before you lose your temper. I know how hard that is. With the ODD, you need to be consistent and firm. If you push too hard, he'll push back. My sister used time outs and body holds to keep temper tatrums from getting out of control.
There is a great national group out there that can help you find great books on ADHD and ODD. They are called National Alliance for the Mentally Ill. Your county mental health center can help you get in touch with them. Or your doc that you get your meds from.
My son has the same problem with the hours from 4 to almost 7 at night. Now that the weather is warming up, we send him outside to play off that extra energy. I know how hard and out of control they can get. Speak quietly, consistantly and use touch to bring him down. Don't do activities that will aggrevate his mood. Is he in school yet? Does he have a consistant bedtime? I know, I've said consistancy a lot but really, that is the key to managing both illnesses. And remember, he's not doing this on purpose. He can't help it. He has a mental illness. I don't mean he's retarded or anything. He is just ADHD and ODD. They are actual problems.
You might qualify for respite care provided by the state and that means the state can pay for your child care. I don't know where you live, here the agency is Childcare Partnerships. They help you with child care.
I also use 1 2 3 Magic. I count to three and if he doesn't do as I ask then, he has a time out. Make him stay in the chair or whatever you use for time out. If he's screaming and trying to get up, hold him in the chair. The timer has to start over it he won't stay in the chair quietly for six minutes.
Another thing I learned was to give my son plenty of warning when we were going to switch activities. Like if he was playing with his blocks and I had to go to the store, I told him we were leaving in 10 minutes for the store and to start putting them away. Then, every minute or so I'd remind him again that he had to put away the blocks so we could leave.
Most kids with these problems don't handle change very well. They need plenty of time to adjust.
Tell him how you expect him to behave in certain situations and they stick to it.
I hope this helps. GOOD LUCK!!!! God bless you for wanting to help him.