Molly O'Brien was born on the U.S.S. Enterprise during the episode Disaster, on Stardate 45156.1. Molly was born during a crisis on the Enterprise, forcing Worf to asist in her birth. A few years later, Molly moved with her father and mother, Miles and Keiko O'Brien, to the space station Deep Space Nine. Molly has also spent a lot of time living with her mother on Earth and on Bajor during the war and during her mother's pursuit of biology.

Molly has a younger brother, Kiriyoshi. Molly enjoys playing with her younger brother. She also likes to color.

In Molly's first two appearances as on infant, she was played by a pair of twins in an uncredited role. In the Deep Space Nine episode Time's Orphan, teenage Molly was played by Michelle Krusiec. Molly was played by Hana Hatae in The Next Generation episode Rascals, and all the Deep Space Nine episodes in which Molly has made an appearance.