She originally worked as military intelligence, but she was later demoted after dangling a colleague out of a window.
She was a big fan of UNIT and the Doctor's work with them, even having knowledge of former companions Rose Tyler, Martha Jones, and Amy Pond, and was thrilled when the Twelfth Doctor finally arrived to help them with their ghost problem.
When the Doctor laid a trap for the ghosts of both their former commanding officer Moran and a Tivolian named Albar Prentis, O'Donnell and Cass stayed back in the control room. O'Donnell was in charge of opening and closing doors so that the other crewmembers could successfully escape the ghosts and lead them to their next target.
When the Doctor decided to travel back to before the flood to uncover the mystery behind the ghosts, O'Donnell and Mason Bennett were the only ones able to get back to the TARDIS with him, and the two journeyed with him back in time. (TV: Under the Lake)
Having travelled back in time to Caithness before it was flooded, O'Donnell joined the Doctor and Bennett in exploring the Fisher King's spaceship before the markings on the wall had appeared. She also met with Prentis, not long before he died. After Prentis' death, O'Donnell was forced to run from the Fisher King, and it ended up killing her and turning her into a ghost.
O'Donnell's ghost appeared in the future along with the other ghosts. She was trapped in the Drum's Faraday cage as Bennett said goodbye to her, admitting he was in love with her. The Doctor told Clara Oswald that UNIT would cut out the Faraday cage and eject it into space where away from the Earth's magnetic field, the ghosts would dissipate. (TV: Before the Flood)