O'Neill began auditioning for parts at age ten with her older siblings. After school, the kids would drive out to Hollywood. O'Neill made her first appearance on television at age 13 in an episode of "Mama's Family" as a younger version of Betty White's character, Ellen Harper. She continued working on television shows such as "Matt Houston", "Night Court", "Highway to Heaven" and "The Twilight Zone". She also appeared on the American game show, "Body Language" in the summer of 1985. After an appearance on "Family Ties", O'Neill won the role of the pregnant teenager Molly Stark on the daytime soap, "The Young and the Restless" for thirty episodes in 1986.