Oleynik (pronounced oh-LAY-nick) was born in Santa Clara County, California, the only child of Lorraine (née Allen), a former nurse, and Roman Oleynik, an anesthesiologist. Her father's family is Russian (although "Oleynik" is a Ukrainian surname) and she was raised in the Russian Orthodox religion. She has cited No Doubt as her favorite band and used to play the piano. Oleynik grew up in the San Francisco Bay Area and began acting in a regional production of Les Misérables, after seeing an audition ad in a newspaper when she was eight years old. After appearing in the musical, she contacted an agent and began taking acting lessons. Oleynik attended Pinewood School and later Sarah Lawrence College, describing the latter as "the best decision I’ve made", and graduated in 2004.