(covers information from several alternate timelines)
She served aboard Enterprise NX-01 during its mission in the Delphic Expanse in 2154. In that year, O'Malley was one of a number of Enterprise crewmembers killed in battle with the Xindi; she was only 26 years old.
Later that year, after Enterprise had traveled back in time to the year 1944 of an alternate timeline, Captain Archer allowed Alicia Travers, a native of the time period, to stay in O'Malley's quarters during her brief stay aboard the ship. (ENT: "Storm Front, Part II")