Beginning her career in summer stock, O'Hara studied acting in New York City, and began performing in commercials and such Broadway plays as Dylan, Promises, Promises, The Iceman Cometh, and The Odd Couple (with Rita Moreno, Sally Struthers, and Tony Shalhoub).
In television, she starred in various made-for-TV movies, including The Return of the World's Greatest Detective, and mini-series like V: The Final Battle. O'Hara also played Thelma Dailey on the soap opera As the World Turns from 1992 until 1993 and had a recurring role as Janet Heffernan (mom to Kevin James' character) on the sitcom The King of Queens. She has made guest appearances on such diverse series as Murphy Brown, NYPD Blue, Law & Order, CSI, ER, Six Feet Under, Barney Miller, and Grey's Anatomy. Film credits include Wishmaster, Mystic River, and Matchstick Men.