Oleynik began acting in a San Francisco production of Les Misérables in 1989 after seeing an audition ad in a newspaper when she was eight years old. She obtained two parts in the production (young Cossette and young Eponine), both with singing roles. After appearing in the musical, she contacted an agent and began taking acting lessons. Her onscreen acting career began at age 12, in a 1993 episode of the television series Dr. Quinn, Medicine Woman; the same year she also appeared in the made-for-television film River of Rage: The Taking of Maggie Keene and in 1996 made a cameo on The Adventures of Pete & Pete as a nurse at the beginning of the episode "Dance Fever". Later in 1993, she was subsequently cast in the lead role of the series The Secret World of Alex Mack where she portrayed a teenage girl who receives telekinetic powers as the result of an accident. She won the role of Alex Mack over 400 other aspirants. The series ran on Nickelodeon from 1994 to 1998 and was one of the network's top three most watched shows, becoming quite a favorite among the child and teen audiences and turning Oleynik into a teen idol. During the show's heyday, children who met Oleynik (and were too young to understand special effects) would often ask her to "morph" for them. Rather than try to explain things, she would quickly glance around, then tell them "Not here – everybody would see!". Oleynik reprised the role in an All That Sketch, although the name was changed to "Alex Sax". She later made an appearance in the 100th episode of the show.