G.B.
If they have been diagnosed with this mental disorder then you should talk to your mental health professional team and come up with a behavior plan. We can tell you all kinds of things to do but if they are not what the team is working on right now then they won't matter.
It is normal for toddlers and preschoolers to try to gain independence by saying no and doing things opposite to what we want. This is a normal stage of development. If it has lasted longer than 6 months though it's probably time to get an evaluation.
It is also odd that your child would be diagnosed this young. It is usually not until they are older and starting school that the actual mental health professionals are called in to do observations and evaluations. If your child had not been diagnosed but you feel they show the criteria for this mental illness then please call a local psychologist is specializes in evaluations and diagnosis so that you do get a valid diagnosis. The medications for this can be extreme and harmful to kids if they are misdiagnosed.
Having a treatable mental illness may mean the child is going to be on meds the rest of their lives so getting it right to begin with is paramount during the diagnosis period.
Here are some good links for ODD information:
This is the DSM 4 criteria for ODD, please note they are supposed to be talking and oppositional. A toddler isn't usually talking. A preschooler is ages 2 up to 5 though and since you put that in I think you mean your child is probably a preschool age child. The article is about a 9 year old boy.
http://www.conductdisorders.com/forum/f7/dsm-iv-criteria-...
This site has numerous links to click on to learn more information. I hope it's a good site to gain a lot of basic information.
http://www.mayoclinic.com/health/oppositional-defiant-dis...
This site has a lot of good information too. It gives a lot of good suggestions for things to do including medications and an eval for ADHD which can't really be done until they are almost school age. They won't give medications to kids under 5 anymore unless the psychiatrist calls the insurance company and discusses it with them. If they can get it approved then they will start them.
http://www.minddisorders.com/Ob-Ps/Oppositional-defiant-d...
I liked this site because it focuses on the effects of the parents parenting style and how that often leads to kids acting out. How that's not a mental illness but a behavior issue. Over permissive parents often end up with kids having behavior problems and they have to learn parenting skills and work to change the bad behaviors of the child too.
http://www.maritalhealing.com/conflicts/angrychild.php
This article talks about how toddlers and small children go through stages of disobedience and how that is not a diagnosable mental illness but a normal stage of development.
http://www.webmd.com/mental-health/oppositional-defiant-d...
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If your child truly has ODD and has been diagnosed with it by a licensed professional they need to work with you on a weekly basis and you'll need to have a good support system in place.
You will be fighting an uphill battle their whole life with the school system and other organizations who want to kick them out due to violence and not conforming to the classroom expectations. They may even need their own aid or assistant to go to class with them to manage their behaviors so the rest of the class can listen to the teacher and learn.
There is also more than a 50% chance this child will be diagnosed with a Conduct Disorder or even be diagnosed in their young adult years with much more serious mental health issues. This is a serious mental health issue. If you are just guessing this is what is going on with your child you need to get evaluations scheduled as soon as possible so that you can start working on a behavior plan now. So that you will have a better change of changing the violence and aggression. If he/she needs meds the sooner they are on them the better too.