O'Brien was born on June 25, 1971, and raised in the small town of Dellroy, Ohio. Her parents were hippie musicians, and her middle name Waterfall was based on the Jimi Hendrix song. The family later moved to Los Angeles, where O'Brien appeared in local theaters. Her mother is actress Jackie O'Brien, with whom she appeared in the film Together & Alone.
O'Brien has appeared in films such as Gas, Food Lodging, Halloween: The Curse of Michael Myers, Being John Malkovich, Diamonds, Ticker, The Mod Squad, and Some Girl. Her television credits include Buffy the Vampire Slayer, Charmed,[citation needed] The Nanny, Once and Again and Courting Alex.
O'Brien appeared on the cover of Spinal Tap's 1992 single "Bitch School".

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