Just a thought, but has she only escalated after the birth of your second child? Since she is 6 she may have been used to getting all your attention and now with another child she may feel like she's not getting enough attention from you.
Have you had her checked out by a doctor recently to see if there are any other disorders with the ODD? A lot of children who have ODD have another disorder along with it. Like mood disorders, learning disabilities, anxiety disorders(if you suffer from anxiety your child may too) etc.
My cousin was diagnosed with ODD at age 7 and was also diagnosed with ADHD a couple months later. His parents did family therapy to help them all and they had him in Psychotheraphy to help control his anger. My aunt was a stay at home mom(around the time he was diagnosed), so she mostly took care of him. The individual therapy helped him a lot, and the family therapy helped my aunt and uncle learn how best to help him keep his anger under control. But it's a continuing thing, he's 10 now and it's still a day to day thing of how his mood is and everything. But they also tried to point out the positive things he does, and if my aunt was on her last straw she'd take a "time out" and explain to him that momma is angry and needs to calm down so she's having a time out(this happend at least once a day with his behavior) but after doing this he eventually learned that if he was really upset he could go off and have his own time out if he needed it. It of course doesn't work all the time but it helps cut down the severity.
Sorry long post but I hopes this helps you some. Good luck and give yourself a break now and again to help keep you relaxed :)
really don't know what to do anymore. -- your words
It's time to re-visit your family doctor for advice re this issue. It is possible that your daughter may require medication in order for her to control her impulsive behaviour. It is also possible that your daughter may suffer from other mental health issues, in addition to ODD. By the way, did a doctor diagnose your daughter with ODD or was that a mother's diagnosis? Just wondering ...