Lullabies is more sophisticated, but less lyrical. It is wittily poignant whereas Oleander is dolorously beautiful and dripping with angst. Lullabies conveys the mystical depth of a child's visual imagination, whereas Oleander's beauty is more verbally poetic.
Oleander strives for sophistication but ends up being overwrought. Lullabies sophistication is a natural offshoot of its simplicity.
O'Neill has an amazing gift for story telling, creative turns of phrase, and depth of insight into the heart of a child.
Go out and get it. You won't be sorry.