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O'Neill enlists in the Colonial Fleet and becomes a mechanic on Galactica, putting her civilian experience as an aeronautical engineer to use. She bonds with Chief Galen Tyrol, and immediately picks up on his relationship with Valerii, stating she knows an "office romance" when she sees one. She is reunited with her husband and daughter, who managed to get off Gemenon aboard the container ship Cybele.
Unbeknown to her, her husband Simon is actually a Number Four model Cylon. He later commits suicide when he is ordered to blow up Cybele by Brother John Cavil, choosing to die rather than hurt the ones he loved and believing his love for his wife Giana would outlast his death. O'Neill is questioned regarding the incident by Colonel Tigh. During the questioning she brings up and then denies the possibility that Simon was a Cylon. Tigh assures her that no one thought he was and conveys his sympathies for her loss. After later learning from Kara Thrace that Simon was indeed a Cylon, Giana discusses the matter with Tyrol. She claims Simon's suicide was heroic, and she'd also kill herself if she suspected she were a Cylon too (TRS: "The Plan"). This later prompts Tyrol to ponder and attempt suicide himself (TRS: "Lay Down Your Burdens, Part I").