In Celie's tenth letter to God, she describes seeing her daughter in a store with a woman. She had not seen her daughter since the night that she gave birth. Her father (and the father of the baby) had taken the little girl from her right after she was born. Celie thought that her father had killed the baby out in the woods. Really, he had given or sold the baby to a reverend and his wife, who happened to be shopping near Celie at the time that she describes in her tenth letter.
She saw the baby and knew right away that it was her child. She writes, "She look just like me and my daddy. Like more us then us is ourself." She spoke to them and examined the girl. "She got my eyes just like they is today. Like everything I seen, she seen, and she pondering it."
There is not much more description of Olivia from Celie's perspective, as she does not get to raise her daughter. Later on in the novel, letters reveal more of what Olivia is like, but Celie is not the one describing her.