She was born in Kraków, the daughter of Polish musician Jacek Ostaszewski of Ostoja coat of arms. Ostaszewska was raised Buddhist. She started her acting training in her native Kraków and later graduated from PWST, in 1996. She has since acted mostly on Teatr Rozmaitości in Warsaw, working with well-known Polish directors such as Krystian Lupa, Krzysztof Warlikowski and Grzegorz Jarzyna.
In 1993, during her acting training, she played a small role in Steven Spielberg's Schindler's List, although her actual real film debut was in 1997 with the TV movie "Przystań." For this role, she received the Best Actress Award at the Polish Film Festival held in Gdynia in 1998.
Ostaszewska played one of the main characters in Academy Award nominated Andrzej Wajda's Katyń.
In 2012, she was awarded "Tele Week" statuettes for her acting, and in February 2013, she received a Wiktory nomination for TV acting.
She is a vegetarian.