Her first job after leaving Drama Centre was in the West End playing Cecily Cardew in The Importance of Being Earnest at the Whitehall Theatre (now Trafalgar Studios) with Hinge and Bracket.
Other musicals include Lust with Denis Lawson at the Theatre Royal, Haymarket, and Bob Carlton's Keep On Running at Birmingham Rep in which she played the lead Kathleen Karman.
[citation needed] O'Sullivan played vet Sally Freeman in two BBC series of Home Farm Twins based on Jenny Oldfield's novels of the same name.
She has appeared in Dead Ringers, Channel 4 film Bill's New Frock, directed by Pete Travis, and was a regular in BBC Scotland sketch show 2000 Not Out with Craig Ferguson.
O'Sullivan's impressions, namely Nancy Dell'Olio and Ulrika Jonsson, featured in Jackson's Sven: The Coach, The Cash And His Lovers for Channel 4.
Written by Sean Grundy and Cara Jennings, it starred Jon Culshaw, Duncan Wisbey as Ralph Bernard and Kate O'Sullivan as Margaret Thatcher.
She was a guest on Steve Punt and Hugh Dennis' sitcom The Party Line for Radio 4 and has played Queen Question in BBC7's Quizland for two series.
[12] BBC Radio 4 productions: What To Do If Your Husband May Leave and Polyoaks[13] the sitcom written by David Spicer and Dr. Phil Hammond, satirizing the government's radical overhaul of the NHS.
O'Sullivan plays various roles in Series 2 of Births, Deaths and Marriages, the BBC Radio 4 sitcom by David Schneider and Simon Jacobs.
Over the first five series, O'Sullivan plays all female voices, characters and celebrities, among them Dolly Parton, a schoolgirl Kate Bush, all 3 Shangri-Las, Gary Numan's bride-to-be and Beethoven's wife.
She has supplied the voiceover for thousands of British television and radio commercials since 1996. Notable campaigns include delivering endlines, impressions or character voices for brands such as Vodafone's Voxi 'Thumbs', Maybelline, Marks & Spencer, L’Oreal, Vision Express among many others.
It won Gold at the ARIAS in The Creative Innovation Award category, for its landmark Absolute Radio 40s event to commemorate VE Day during the pandemic.
Other cartoons: Treasure; MechaNick; Audrey; and Castle Farm series 1 & 2 (Channel 5), in which she characterizes Farmer, Cow, She-Ba, and Winnie the Hen in the children's animation.
[29] She is also the voice of Professor Peabody in Orion's 2008 audio version of cult comic strip Dan Dare- Voyage To Venus by Frank Hampson, with Rupert Degas, Tom Goodman-Hill and Christian Rodska.
O'Sullivan is part of the writing team for Tonight/Bremner's One Question Quiz (BBC Radio 4, 2011–2015) with Rory Bremner, Andy Zaltzman, Nick Doody, John Langdon and Geoff Atkinson.
On winning the competition, for which 900 other hopefuls had auditioned, she was given an Aiwa sound system, a signed photograph of Bernie Clifton and a coveted Crackerjack pen.