I can understand why you are concerned about how your child got it. But, it really isn't important. What is important is how you work with your child to overcome this.
I assume this is a diagnosis from a psychiatrist and not a pediatrician? Have you looked up ODD. Good place to start is here - http://aacap.org/page.ww?name=Children+with+Oppositional+Defiant+Disorder§ion=Facts+for+Families
It is important to note the statement, "It is important to look for other disorders which may be present; such as, attention-deficit hyperactivity disorder (ADHD), learning disabilities, mood disorders (depression, bipolar disorder) and anxiety disorders. It may be difficult to improve the symptoms of ODD without treating the coexisting disorder." In the book, "The ADD/ ADHD Answer book," by Susan Ashley, she states that 33% of the kids with ADHD also have ODD. How you work with a child really depends on what the other disorder might be. So it is really important to pin that down. Hope this helps.