If I-330 is all sharp and predatory, O-90 is all softness and gentility:
She has a round soft dimple on her wrist. Children have such dimples. (2.4)
In the book, she's presented as very happy and open, at least at first. She's quite loyal to the State, but D-503 speaks of her in kind and gentle terms. This is intended in part to contrast her with I-330—the hard- edged rebel vs. the gentle conformist—as well as giving D-503 a real-life figure to pull against I-330's influence.
Alas, O-90 hides rebellious tendencies under her surface, like a lot of characters in this book. Unlike many of them, her rebellious instincts come from politics, but from biology. She wants to be loyal to the State, but she also wants a baby. In the end, her desire for motherhood wins out. Pay attention to how she talks about impending mommy-dom:
"I want, I must have a child! From you! Give me a child and I will leave. I will!" (19.29)
In becoming a mother, she becomes an enemy of the State and is forced to flee with her child beyond the Green Wall. Once again, a very human urge—to procreate—wins out over all the indoctrination of the State. As with D-503, O-90's character seems to be telling us that human nature will win out every time.
The sad thing is, in this world, that's the closest she'll get to a happy ending. She's on the run in the wilderness, but she has friendly forces there to help her and she'll be able to raise her baby in peace. Contrast that with everyone else, whose either dead of lobotomized.
O-90 represents hope for a better tomorrow. Her instinctive warmth and gentleness speaks to everything great in the human soul, everything the State is attempting to wipe out. That allows her to slip away while others die fighting, staying alive to carry on the hope that all those other characters gave their lives for. She's a hope repository, which matches her soft round face, and reminds us that some people always care and feel even in a place like the One State.