She lived at Omicron Theta, where she married Dr. Noonien Soong. Juliana assisted Soong with his research and experiments, and considered herself to be the "mother" of the androids Lore and Data (and presumably B-4). When the Crystalline Entity attacked, O'Donnell was seriously injured and slipped into a coma.
Dr. Soong transferred her memories into an android meant to be indestinguishable from a human. O'Donnell, remaining unaware of her new state, became frustrated with Soong's devotion to his work above all other things and left him in 2365. (TNG - Strange New Worlds VII short story: "Life's Work"; TNG episode: "Inheritance")
Soong programmed the Juliana android to appear to die of natural causes after a natural human lifespan. This happened in the year 2374. Juliana was soon reactivated with full knowledge of who and what she was by the immortal known as Flint. (TNG novel: Immortal Coil)