If your difficult child is on a stimulant like LDM suggested for ADHD, I'd think it would curb her appetite more, not make her eat. So, if she's not on something for her ADHD (i.e. Ritalin, Concerta, Metadate, etc), might be something to consider.
I have a child ADHD, Combined Type and a child Bipolar. My ADHD son is on Concerta, and RARELY eats. However, he is also extremely active, the first one outside to play, ride his bike, etc, and doesn't like sitting on the couch in front of the TV.
My difficult child, Dylan, who is Bipolar, on Bipolar medications, eats like a cow. Even if it's celery, he'll eat 3x more than a normal person, just to eat. He eats when he's bored, he eats when he's hungry and he eats just to eat. At almost 11, he's 120 pounds. Obese. I blame 80% of this on the medication (Lithium), and 20% on his/my eating habits (I'm the one who buys the junk).
I would also try to redirect her. This works well for Dylan. So, for instance, if she's bugging you for something to eat, grab a board game, some crayons and coloring books, or take her outside.
Neither of my children are ODD, and I haven't heard kids with that label, specifically, being overeaters.
