Keep in mind that Oracle's Marks are hard to come by, so you might want to wait before reclassing her. Despite the setbacks, she still has access to Nature's Touch and Summon magic, giving her high damage potential despite her stats.
However, like her sisters, Olivya has access to the exclusive Shaman class, lending herself to Water Magic in particular she can be a great Shaman, though her INT is a bit low and the Water school of magic is arguably the least useful. However, you may already have several Clerics in you party (and possibly be drowning in them if you are on the Neutral route), Olivya joins as a Cleric, a class she's very good as.
Overview Tactics Ogre: Let Us Cling Together (PSP) She joins automatically after visiting Brigantys Castle near the beginning of Chapter 4, no matter how you went about it. It's heavily implied that she's been in love with him since childhood. She is particularly close to Denam who she grew up alongside and is close in age to. She's also the most religious of the four and continued her father's teachings while her sisters became more involved in the political strife and violent upheavals. Of the four Phoraena sisters, Olivya is the most level-headed and even-tempered, possessing a compassionate spirit as befitting of a sibyl. She then joined the Resistance in the hopes that it would lead her to her own father. When Denam came to the castle she arranged for them to meet, and later told him of his lineage. She came upon Abuna Prancet while traveling the Corpsevale on the outskirts of Heim, and took the ailing abuna to Brigantys Castle. She joined the upper echelons of the Order upon her father's disappearance, and her stoutness of heart garnered much trust from the congregation. After escaping political strife in Heim, she continued to pass on the teachings of the Order with her father. She is the daughter of Archiereus Mreuva.