Oleynik began her career as a child actor, first appearing onstage as young Cosette in a national touring production of Les Misérables (1989–1991).

Oleynik subsequently had a supporting role in the comedy 100 Girls (2000), after which she starred opposite Nastassja Kinski and Scarlett Johansson in the period film An American Rhapsody (2001), and the independent drama Bringing Rain (2003).

[8] As her acting career flourished, she would "divide her time between normal childhood experiences in Northern California and auditions in Los Angeles.

[13] After appearing in the musical, she was referred to an agent by her Les Misérables co-star, Rider Strong, and began to take formal acting lessons.

"[12] Her onscreen acting career began at age 12, in a 1993 episode of the television series Dr. Quinn, Medicine Woman;[12] the same year, she also appeared in the made-for-television film River of Rage: The Taking of Maggie Keene.

Later, in 1993, she was cast in the lead role of the series The Secret World of Alex Mack, where she portrayed a teenage girl who gains superpowers in a chemical accident.

[15] Also during her time on The Secret World of Alex Mack, she played Dawn Schafer, one of the lead characters in the 1995 feature film The Baby-Sitters Club (1995), opposite Rachael Leigh Cook and Schuyler Fisk,[16] appeared in several episodes of Boy Meets World, wrote an advice column for Tiger Beat magazine,[17] and was involved in Nickelodeon's The Big Help charity, Hands Across Communication, Surfrider Foundation and the Starlight Children's Foundation.

Oleynik had a supporting role in the independent drama film An American Rhapsody (2001), opposite Nastassja Kinski and Scarlett Johansson, which follows a young woman whose parents are forced to leave her behind in Communist Hungary while they flee to the United States.

[20] Oleynik had a supporting part in Bringing Rain (2003), a low-budget teen drama starring Adrian Grenier and Paz de la Huerta, which premiered at the Tribeca Film Festival.

[24] She was subsequently cast in the Ayn Rand adaptation Atlas Shrugged: Part II (2012), as Cherryl Brooks, a store clerk who becomes acquainted with the protagonist, Dagny Taggart (portrayed by Samantha Mathis).

[25] Oleynik also guest-starred as Ken Cosgrove's girlfriend (and later wife) Cynthia Baxter in several episodes on the AMC television series Mad Men.