Serin Osman was born in 2511 as Serin Çelik to a single mother on the human colony of Cascade. In 2517, she was kidnapped for inclusion into the Spartan-II program led by Doctor Catherine Halsey and under the authority of the Office of Naval Intelligence. Osman, known as Serin-019 throughout the program, completed the Spartan-II training, but washed out after the physical augmentations the Spartan-II candidates were forced to endure, were unsuccessful in her.
After the Spartan-II program, ONI brought Osman back to full health and she became the protege of then-commander-in-chief of ONI, Margaret Parangosky. Shortly after the Human-Covenant War, Osman was put in charge of a top-secret ONI team known as Kilo-Five, who were tasked with worsening the Sangheili civil war and weaken the Sangheili power in the galaxy in favor of the UNSC. Their mission was ultimately successful.
Sometime between 2553 and March 2555, Osman became the commander-in-chief of ONI, replacing Parangosky. As head of ONI, Osman took a more active role than her predecessor, particularly when it came to the actions of the UNSC Infinity. Her order to Captain Thomas Lasky to kill Doctor Halsey in 2557 was responsible for the doctor being shot in the arm and ultimately having the arm amputated.