Doctor Who was O'Brien's first acting job on television after graduating from drama school. She was the first "new" actor to join the regular cast, after the departure of Carole Ann Ford as original companion Susan. Her time on the show began with "The Rescue" early in Season 2. It was brought to an abrupt end early in Season 3 after she criticized the script for the season's first serial, "Galaxy 4". Unhappy with her complaints, producer John Wiles decided not to take up The BBC's option to renew her contract, and instructed that she be written out of the series when it expired with the next serial, "The Myth Makers". O'Brien was taken aback to discover this when she read the script, as Wiles had not bothered to inform her of his decision.
After leaving Doctor Who, O'Brien enjoyed a long acting career in British television over the following decades, her final onscreen credit coming in 2003. She also branched out into fiction writing, penning seven detective novels between 1989 and 2004, all featuring the character of Detective Inspector John Bright.
More than four decades after her unceremonious dismissal from Doctor Who, O'Brien reprised her role as Vicki in audio form for Big Finish Productions from 2007 onwards. She has now appeared in over twenty of their audio plays, usually as Vicki, but she has also voiced the character of Agnes Landen in the Dalek Empire series and Alice Bultitude in "The Year of the Pig".
Her roles include:
Television
- Doctor Who, 38 episodes / 9 storiesshow list (1965) — Vicki
- Z Cars, episodes "Not That Sort of Policeman": Part 1 & 2 (1969) — Nora Leach
- Bergerac, episode "Relative Values" (1981) — Joan Hadley
- CASUAL+Y, 15 episodes (1987) — Elizabeth Straker
- Taggart, episodes "Forbidden Fruit" (1994) — Ruth Millar
- Jonathan Creek, episode "Jack in the Box" (1997) — Kirsten Holiday
- The Bill episodes:
- "Coming to Terms" (1993) — Pat Tolman
- "Other Voices" (1995) — Joy Grainger
- "Sparks" (1997) — Margaret Ames
- A Touch of Frost, episode "Private Lives" (1999) — Marion
- Doctors, episode "Good Grief" (2003) — Doreen Crossland
Audio Play
- Big Finish Doctor Who stories:
- Doctor Who Main Range: "Year of the Pig" (2006) — Miss Alice Bultitude
- Dalek Empire: "The Fearless" Parts 1-4 (2007) — Agnes Landen
- Companion Chronicles: 7 storiesshow list (2007-19) — Vicki Pallister
- Special Release "The Light At The End" (2013) — Vicki Pallister
- The Lost Stories: "The Dark Planet" (2013) — Vicki Pallister
- The Early Adventures: 8 storiesshow list (2014-18) — Vicki Pallister
- Doctor Who Main Range: "The Secret History" (2015) — Vicki Pallister
- Doctor Who: Once and Future - the Union (2023) — the Union