She later made an appearance in the 100th episode of the show.
Also during her time on The Secret World of Alex Mack, she played one of the lead characters in the 1995 feature film The Baby-Sitters Club (opposite Rachael Leigh Cook and Schuyler Fisk), appeared in several episodes of Boy Meets World, wrote an advice column for Tiger Beat magazine, She has also hosted the Cable ACE Awards, Daytime Emmy Awards, YTV Achievement Awards, The Nickelodeon Kids' Choice Awards as well as The Big Help.
The film was released in April 1999 and did fairly well at the box office, grossing a total of $38 million domestically.
From 1998 to 2000, Oleynik appeared in twenty-one episodes of the NBC series 3rd Rock from the Sun as Alissa Strudwick.
She won the role of Alex Mack over 400 other aspirants.